Why does my verizon phone go into service mode instead of charging?

Lat night i was about to go to bed, so i went to plug in my phone to charge like i always do. I have one of them verizon phones where the bottom turns from like some mp3 controls to a keyboard. anyway, when i plugged it in to charge, it went into service mode and wouldnt charge, and i had to power cycle my phone to use it. Why is it doing this? And, more importantly, how do I fix it?