So you've held power+volume down until it shuts down, and that hasn't helped.
OK, in that case my next guess would be to clear the system cache or try a factory reset. If this is a problem with corrupt data it should fix it. This won't help with corrupt system software, but unless you had a failed system update that shouldn't happen, and won't fix a hardware problem.
To do either of these, reboot while pressing the volume up and volume down keys. You should get a boot menu. Select recovery, then from recovery you can clear cache or erase data/factory reset. I'd try clearing cache first, as it will be a lot less painful. The factory reset is a last resort, because I've read that on Motorolas a factory reset done this way will erase everything on the device. To be safe I'd remove the sd card before doing this (that may be OK anyway, but why take the risk?).
Please note, I don't own this device, so am relying on what I've been able to find out. I compared a couple of sources, so hopefully the information is accurate
