Corrupted game files may cause a variety of issues. If you have encountered such a problem, please verify the integrity of the game's files.
From the Library section:
A) Right-click on Insurgency: Sandstorm
B) Select "Properties"
C) Select "Local files"
D) Click "Verify integrity of the game files..."
Steam will then start verifying the game's files for any missing or corrupted content. This process may take several minutes.