For anyone affected by this, a simple trick to reviving a Sansa Clip+ that won't turn on is to hold the power and home button simultaneously for about 20-30 seconds. Let go and press the power button and it might turn on.

I'm not sure what causes these players to become unresponsive. It's happened to me when it tries to play an unusual file or after I've transferred music to the device and left it plugged into the computer for a while.

They are still my favorite standalone portable mp3 player, even with the stock firmware.