As Douglas Adams would say, DON'T PANIC! File an amended return, obviously.
The Golden Rule here is: if you tried to do things right, you're not going to prison. Maybe just a beat-down and a night in a holding cell. Just kidding, you're fine. You're not the first person to inadvertently leave something off your return.
Get your amended return in by April 15, and pay the difference, and there will be no penalties or interest. Just like if you did it right the first time. Well, except for the damage from the tear gas they fire through your windows.
If you fail to do that ... well, the IRS no longer sends dobermans into the houses of tax-cheats like you. They'll most likely send you a bill and charge you a perfectly reasonable interest rate. They do still send nasty letters. Take these seriously, but DON'T PANIC!