I had this problem and the only way to solve a frozen screen is to let the unit run out of battery and bring it back to life. Battery will die out on you for 6~8 hours. Once battery is dead you must do the following.
Waking up your c330...
If you left your Garmin c330 on running
the battery will have 1% life left. If you turn c330 back on again, it
will flash screen for 3 seconds then die, even charging it at this
point will not wake it back up. TO BRING IT BACK TO LIFE... plug in the
power home or car into Garmin c330. There is a tab in the front Garmin
gray frame of your screen. You must take like your fingernails and
under the logo "GARMIN" pull the whole gray plastic frame off.
Underneath the plastic frame to the right hand side. There is a "RESET"
button. Make sure power is plugged in while hitting reset. Your battery
should come back to life. Cheers