I'e never had that happen to my Kenwood MP828, although I only had it a short time. < 6 months. Sadly, 99% of the time a car stereo isn't worth fixing. It's generally not cost effective in comparison to just buying a new one.

One thing you could try is completely disconnecting it from power for a few minutes. BUT, make sure you have the four digit security code if your HU is so equipted which I'm sure it probably is.