Fortunately, steam loves you.
Right click [broken game] In the steam games list > Click properties > Select the "local files" tab > click Verify integrity of game cache.
If there's problems with your game, this'll check if it's a file issue, and redownload whatever is broke.
However, sometimes this doesn't work, you can try deleting your game's folder. For example, if your Portal is broke, delete the folder called
Code: Select all
C:\Program Files\Steam\steamapps\email@badgers.co.uk\portal



