So it doesn't drive?

Check CAN cables! If ANY of the CAN connectors come undone, the CAN bus will go offline, taking ALL motors down with it and you will have a very fun time trying to drive the robot, especially if you are in a match.

Is it smoking? If it is you better turn it off and unplug whatever was smoking. DO NOT SNIFF IT.

Check battery voltage! Brownout protection is extremely annoying.

Did you select the right mode? If you selected autonomous and enabled it by accident, pray that you haven't put a hole in the wall.

Check your motor controllers! If you put a screw that is too long into a hole that is too short, it will not end well.

Does the code work? Run the code in robot simulator and see if it compiles/crashes.

Check PDP voltage: Should be 12v. If it isn't then charge the battery. 

Recheck all connectors. Especially if you have unmarked cables. There is always the possibility that something is crosswired.

Is E-Stop triggering? If it is randomly triggering you probably need to restart driver station.

Is the robot powered on? (do I even have to explain this?)