The most common cause of this is corrupt files in the user directory or missing prerequisite files.
Some players may experience issues with receiving drops in game if they have security software such as an antivirus or firewall blocking the game.
Your monitor goes to a black screen and/or gives you an error message (similar to out of range, no input, etc) the most likely cause is the game is attempting to render at a resolution your monitor is not capable of supporting.
If your perks have appeared to reset to 0 completely, it is likely that your game client is having trouble reaching the Steam backend where your stats are saved.