It seems to be a specific problem arising from having created a custom named folder for Steam installs that you did not want installed to your default Steam library folder. Usually the default folder is C:/SteamLibrary, and if you want your games on D: instead, Steam will provide to select a default folder in a drop down menu, and it will be named for you as per D:/SteamLibrary.

If you chose the option to install games to a specific folder in a specific drive, however, and if that folder's name is not the default name "SteamLibrary", the client apparently might forget what that folder was named and subsequently the entire drive will be forgotten along with the install folder.

Here's how you might be able to fix this error and have your games up and running in just a few simple steps, without needing to weed out complicated uninstalls (because Steam can't even uninstall the affected games if this happens, "lol") and without having to resort to lengthy re-installations of your big games.

Open your Steam settings and click Downloads, then select Steam Library Folders.

Here you should notice that the drive in where you've installed your currently inaccessible Steam games, is missing from the Storage Manager. Click on the small + icon at the tip of this arrow.

You will now have the option to "Let me choose another location", select that.

When you have clicked on "Let me choose another location", your choice will be confirmed as selected and an option to "Add" a folder/location will pop up in a blue button beneath this selected choice.

Another window will open where you have a drop down menu, select the drive that contains the installation folders for your Steam games. In my case I had to choose the D: drive.

When you've selected the drive in which you have your Steam games installation folder, mark the folder so it becomes highlighted/selected (7), and then click on the button that says SELECT (8).

When you've pressed SELECT in Step 8, you should be sent back to Storage Manager, and you should notice that the drive in which you have your additional Steam library has been added back to it. When you click on the Drive, a list of your currently installed games on that drive should also appear in the Storage Manager window.

Hopefully now your Steam library will be updated with proper launcher directions, and your games that were inaccessible just a few minutes ago will now have the big, green PLAY button on them again, instead of the install button.