They're prone to just stopping working like that! I've had two die on me. It's highly doubtful that the speaker connectors have come undone, unless you've been fiddling around behind there. On mine the chief culprit was the amp (or some internal connection) and the cost of a replacement one off eBay was less than the cost of getting it fixed. People seem happy to buy broken ones, so could be worth selling yours on and getting a new one - or get a cheap single DIN headunit which plays MP3s (so you can play burnt CDs) and a fascia adapter to make it fit!