Something else that causes a problem like that is having the computer sleep or shutdown while NEM is running.  Sometimes when the computer starts back up, NEM is fine but sometimes it isn't.  In that case your database can be corrupted and you have to follow this guide to remove your database and sync a new one from scratch (luckily that usually doesn't take too long).  http://blog.nem.io/how-to-delete-your-local-copy-of-the-blockchain/