If you attempted to use 20 invalid (or used, obviously) codes in one day, you're temporarily "locked out" for a day of entering codes on MCR. It's happened to most of us who've been doing it for some time, especially when you have a code with a fuzzy/missing letter or two. You just keep trying to find the "right" combination and, whoops! All of a sudden, you can't enter *any* kind of code, even ones you *know* are good.
Don't lose any sleep over it. If it's not working by Thursday afternoon, give it until early morning (read: 12:01AM EST) Friday. If you're still not able to enter codes that you *know* are valid, then it may be time to call MCR and find out what's what. For now, just relax for 24 hours. It should resolve itself. It has for me on multiple occasions.