When the phones boot off of the pbx, they will adopt the Allworx phone admin password located under SERVERS->VOIP in the PBX admin console.
1. Reboot the phone.
2. As soon as you see the "Configuring Network" on the display, press the RELEASE key 3X.
3. In firmware 7.1.x.x and later, this will abort the boot up process.
4. At this point the phone's admin password should be 2559679. Enter this and press the check mark (enter) key.
That should do it.
And, no, if you have a phone that is truly bricked where you get no display and/or a couple BLF/PFK lights lit solid and no further activity, the phone is done. You'd have to RMA it.