Try the reset procedure from the BackTrack FAQ at http://line6.com/support/docs/DOC-4769:

To perform a hard reset you'll need to manually press a small object like a paper clip into the small hole on the side of the Backtrack. The hole is located on the opposite side of all the UI controls (Backtrack) or on the side opposite the microphone (Backtrack + Mic). Hold the button down for three seconds and then remove the paperclip. The LED on the device will glow white, blink a few times, and then begin blinking blue. You have now successfully reset your Backtrack.

Also, make sure it's charged by leaving it connected to your computer for a few hours.