Ah, sorry I should've warned you to turn off the phone. In a bootloader state (failed to boot screen) the phone doesn't charge with end user cables..there are numerous options:
Find a friend with an Atrix (borrow his/her battery or phone to charge your battery)
Find an AT&T store with an Atrix to charge in the display or they sometimes have battery chargers.
Droid X's have the same battery so the same as above options.
And then there is a engineer method where you strip a chord down to charge the phone directly (I can link you if you are interested).