Possible problem with teh switch on the headphone jack wooould be my guess. When you plug in the headphones it opens a switch to disconnect the speaker. Unfortunately, these jacks are weak and often the switch fails to close when the hedphones are unplugged.

You need to open the amp and check if you can close the contact by pressing on it. There are several designs of these jacks and MOST are weak and having a cord plugged in and tripping over it is SURE to break the jack.

Here is a site showing jacks for a Fender, but most are similar.
Look at the one with several strips of metaal between the little black walls... they are NOTORIOUS for the switches not working. OFTEN you can bend the springs tigter to restore them. The site with pix in a box at the top:

http://www.catalinaguitars.com/catalog2/index.php?cPath=66_86