A Honda radio will stop working if the battery is completely disconnected. A dead battery tends to have enough juice to power the radio memory, so if you merely get a jump start, your radio should still work. If your battery has been disconnected or replaced, you'll need a code to get it to work correctly again.
Make sure there is not some other problem. Many times, a circuit fuse can blow, causing the radio to disconnect from the power supply. If a blown fuse is the problem, simply typing in a code won't help you.
Find your manual. The car manual is usually in the glove box, but if you put yours somewhere else for safe keeping, dig it out. The code is in the manual. Most Hondas do not have it inscribed inside the door or under the hood, as some other vehicle types do.
Check the very last page. You should see an antitheft ID card, or a sticker in the back where you'll find the code. If you don't have the code, in vehicles newer than the 2003 model, your manual will give you instructions as to how to prompt your radio to give you the audio unit serial number. If you have that, a simple call to Honda will get you the code. Otherwise, you'll have to visit a dealer.
Punch the code in, using the buttons for station favourites. The code should be five numbers long with the lowest available number being one, and the highest available number being six.
If you enter a wrong code too many times, your radio will lock, forcing you to go to a dealer to get it unlocked.