I had the same problem. This procedure from the "Technical Questions" section of http://shopdelphi.com/consumers/support/faqs/myfi/
worked for me:
"My MyFi won't power up, how do I reset it?
To reset your MyFi, start by turning the receiver off and removing it
from its (car, home, audio system) cradle. Then remove the back cover
and battery. Using a small paperclip, press the reset button located on
the top left corner of the battery compartment. Now plug the AC power
adapter into a wall outlet and plug the power port directly into the
bottom of the MyFi (battery stays removed). Wait 10 seconds and listen
for a beep. After you hear the beep the message "insert battery" will
appear. Cycle the MyFi to ensure it powers up and powers down. If the
power cycle is normal, then turn off the MyFi, unplug the power adapter,
and reinstall the battery. Re-plug the AC power adapter directly into
the bottom of the MyFi. Wait 10 seconds and listen for a beep. After the
beep you will see the message "battery charging". Leave the MyFi
plugged in until it's fully charged. You will see the message "charging
complete" displayed. After the MyFi is fully charged, turn it on and
reset to the factory defaults. Unplug the MyFi from the AC power adapter
and using the battery power leave it on until it fully discharges
(approximately 2-5 hours). Plug the MyFi in using the AC power adapter
to recharge the unit and resume normal use.
If you need help at any point during this process contact us at
1-877-463-3574 for technical support."
I hope it works for you too.