(The steps in this guide may not work as the game has changed quite a bit since I wrote this guide and I haven't had any issues since I last played the game. The steps in this guide might still work but I have no interest to check or update the guide. Besides, I think this issue was mostly patched out. I've left the guide up in case you still want to try any of the steps.)

Recently some maps were giving me a "AI Disabled" message to the right of my screen. Annoying enough as it is, NPCs stopped working. I've managed to fix the problem by playing around with the developer console and these might just work for you. These steps may work in other Source games but I can't guarantee they will.

First things first, if you don't have your console enabled, you'll need to enable it. On the main menu, click options and go to the keyboard tab. Then click the advanced button at the bottom of the window and check "enable developer console". Check it is working by pressing the '`' key on your keyboard. If nothing appears make sure the key is binded correctly or try bind it to another key.

This is the one that worked for me. Load up the map having problems, and create a save. This is important otherwise you might end up losing game progress. Now open up the developer console and type "nav_generate". You'll see the console starting to output some things, but you don't need to worry about it. This shouldn't take too long but it really depends on how fast your computer is. After it is done, the map will reload. You will probably end up at the start of the map with no HEV suit or any weapons, just reload your save from before and you shouldn't have the AI Disabled message anymore. If it happens with other maps, just do the same thing.

This didn't do anything for me but if the above doesn't work, try this. Create a save, open the developer console and type "ai_disable" but don't press enter yet. You will likely see that the number is set to 1 and, if it is, add a space, replace the 1 with a 0 then press enter. At no point does the game set this command to 1 (or at least what I'm aware of it doesn't) which leads me to believe the cause of this problem is custom maps, but I could be completely wrong.

Do the same steps as the last two; create a save, open the developer console and type "ai_resume". This should resume the AI and the game should be playing normally. I haven't tried this one, but give it a shot.

Black Mesa's save file system was updated a while back and older saves started to become incompatible. They would either refuse to open or events would become screwed up. If you don't mind losing some progress you can start a new game and pick the chapter you are on. I had to do this because when the game stopped to load the next part of the map, it would crash.

Steam has a built-in function which can check your games for missing/corrupted files. If a corrupted or missing file is causing the issue, Steam will most likely be able to fix it. To start this process, right click on Black Mesa and go to "Properties." Go to the "Local Files" tab and click on the button that says "Verify integrity of game files". Steam will scan the game directory for files that fail to validate when being checked and these files will be fixed. This may take a while depending on the amount of game files, your computer's speed and possibly your internet speed as damaged files need to be replaced with fresh copies from steam.

If you are still getting the "AI Disabled" message try reinstalling Black Mesa. Before you do backup your save data by doing the following:
1. Go to where Black Mesa is installed. If you haven't changed any of the default locations, it will most likely be in the following:

Windows: "C:\Program Files (x86)\Steam\steamapps\common\Black Mesa" or "C:\Program Files\Steam\steamapps\common\Black Mesa" if you're using a 32 bit system.

Linux: Use a file browser or terminal and go to: "~/.local/share/Steam/SteamApps/common." The '~' is short for "/home/YourUsername/". I don't have Steam installed on any of my Linux installs, so let me know if this is incorrect.

2. Go to the "bms" folder, find the folder that is called "save" and move it to somewhere such as your desktop.

3. Find a file called "gamestate.txt" and move this to the same location.
4. Remove Black Mesa by right clicking on its tab in steam, then clicking "Uninstall".
5. Now reinstall Black Mesa and when it's done move both of the files back to the new Black Mesa installation.

Tip: (Please note this way of backing up saves isn't always realiable.) If you don't want to go thorugh the process of moving your save files, Steam Cloud is also an option. Right click on Black Mesa then choose "Properties". Click on the "Updates" tab then make sure that "Enable Steam Cloud synchronization for Black Mesa" is checked. You may need to start your game and create a new save for your current saves to sync correctly. To check if they have synced correctly, look under the "Steam Cloud" section of the updates tab. If the size is not something along the lines of "0.00B stored / (X) avaliable", you should be good to go. Don't forget to enable synchronization once you install the game again, so you can get your saves.

If the AI is still disabled after doing this, try steps 2 and 3 again as your save file might still have the commands such as "ai_disable 1" stored.

If none of the above steps worked, it's most likely a bug in the game (or custom map) the developers can fix. If it's a custom map, tell the creator about the problem. If it's in Black Mesa itself, contact the developers on their forum page or their Twitter account. You can access them here:

http://forums.blackmesasource.com/
https://twitter.com/BlackMesaDevs

Make sure to give them as much info as you can, for example, when the problem started to happen and the hardware you're running the game on. This would include things such as your graphics card.