If your tracker has been inundated with text messages or phone calls from you while it was in a low to no reception area, it can lock up the tracker. The several messages it needs to send through before it can stop processing can flood the units processing power.
The following command may help to reset it:
Last resort would be to power your tracker down for 5 minutes.
NOTE: If your password differs from 123456 then send your password of course.