Hi there,
If the screen is frozen, the first thing that I would try is doing a hard reset on the unit and then loading on the latest firmware for the unit.
Doing a hard reset will unfortunately lose quite a bit of the data on the unit, but it does free up memory and usually allows you to get started again.
It's also possible that after a hard reset that you may have to reload the maps onto the unit so make sure that you have a copy of the unlock code for the unit to hand
To hard reset the 2820, do the following:
Hold down the Map and Menu buttons when the unit is off and switch on the unit.
The latest Garmin firmware for this unit is about a year old and is available here:
Garmin: StreetPilot 2820 Software Update Collection
HTH,
David