If your game often crashes or you noticed some issuses/problems it can have many reasons. First check if your computer is able to run Left 4 Dead 2 (minimum requierements:
OS: Windows® 7 32/64-bit / Vista 32/64 / XP
Processor: Pentium 4 3.0GHz
Memory: 2 GB RAM
Graphics: Video card with 128 MB, Shader model 2.0. ATI X800, NVidia 6600 or better
DirectX: Version 9.0c
Sound Card: DirectX 9.0c compatible sound card
source: http://steamproxy.net/steamstore/app/550/).

Nevertheless some people have sometimes crashes or missing/corrupted files for example after playing on a heavily modded server.

The following guide will help you to minimize the errors/issuses/crashes by using Steam´s option to verify the game caches of Left 4 Dead 2.
As I know this process does not help to solve problems with addons.

English:

Right click in steam on Left 4 Dead 2 -> properties -> local files -> verify integrity of game cache...


German:

Rechtsklick in Steam auf Left 4 Dead 2 -> Eigenschaften -> Lokale Dateien -> Spieldateien auf Fehler überprüfen...


French:

Clic droit sur le Left 4 Dead 2 -> Propriétés -> Fichiers Locaux -> Vérification de l'intégrité du cache du jeu...


Spanish:

botón derecho del ratón en Steam en Left 4 Dead 2 -> propiedades -> archivos locales -> Verificar Integridad de Caché del Juego...


Italian:

In Steam click col tasto destro sul Left 4 Dead 2 -> Proprietà -> File Locali -> Verifica integrità della cache del gioco...


Russian:

Правый клик по Left 4 Dead 2 в стиме -> свойства -> локальные файлы -> Проверить целостность кэша...


Portuguese:

Clique com o botão direito no Left 4 Dead 2 -> propriedades -> Ficheiros Locais -> Verificar integridade da cache do jogo...