Here is the procedure to calibrate the screen if you can not reach the calibration screen through the GPS menus.
Nuvi 2xx/2xxW Series: Turn the Nuvi off. Place your finger on the upper left-hand corner of the screen and then slide the on/off power switch to the left and hold until you see the calibration screen. Note: Some 2xxW models require the procedure below for the 7xx.
Nuvi 7xx Series: Turn the Nuvi off. Touch the screen anywhere and keep your finger there while moving the on/off switch to left (don't wait for Garmin logo to come up) and then quickly moving the switch the right (lock position). After a short while, you will reach the calibration screen. Go through the entire procedure of pressing each dot as it appears on the screen until it says "Done".
Now if you go through and press the dots and it keeps looping without saying "calibration complete" then the touchscreen has drifted out of specification and must be replaced.
Please see this link for more information about touchscreens and how to get it replaced.