There is a stem on the end of the cap that has marks on it to check the fluid level. If the steering pump is not pumping fluid and the power steering is now manual steering it's quite possible your pump is bad.
Overfilling the pump would not break it. It would just result in a large fluid mess as the excessive fluid would be pushed out the cap. If the engine was ran long enough with the fluid level low that could have ruined the pump. Allow the fluid to settle...engine off...then recheck the level running and see if you have any assist. If you still do not have any assisted steering you will want to have the system checked. If the system is moaning, cycle the steering back and forth several times from stop to stop. Recheck the fluid level until full. This should bring back the steering assist, otherwise you may need a pump, a steering gear, or possibly both. Hope this helps.