When you say the screen has frozen - do you mean the map has stopped moving, or the whole device has locked up?
Either way, before you do that hold the corner of the screen thing and power on (which will wipe all your 'user data' or so it appears - they do come back again) I would remove the battery and let it sit for a couple of hours... that often does the trick.
If it's specifically the map which is not moving, then again, remove the battery, and also check your SD card is lined up properly in the slot (they can rattle loose, especially on rough terrain) and consider sticking a piece of sellotape over the hinge/SD card to stop it moving in future.
Jx
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