Hey,
I have an accord and I have not run into this problem because I put a new stereo in my car. But my friend who has a Honda Civic has had this issue.
He found out that each radio comes with a "CODE" that is in place in case the radio is stolen from the car. But as we found out if your battery dies or the radio is disconnected from the power you must enter the code, it thinks it has been stolen.
My friend fixed his by contacting the dealer he bought it from and they had it on file so they just told him and the problem was solved. So my best advice is to contact the previous owner or honda and see if they can help you out. You can also check in your manual or paperwork for the code, it may be written somewhere.
Best of luck.
~Knace
>> Stay informed about: radio shows ""CODE""