Have you got any protective screens in place - they can sometimes confuse the touchscreen layer if too thick, and should be removed if you are having problems.

If you don't have a protective layer or removing it makes no difference, then...

Step 1. Pull the battery for 5 seconds then power up the device

If that doesn't work...

Step 2. Reflash the device using the last known good firmware

If that doesn't work...

Step 3. Return to Nokia (warranty repair) or place of purchase (refund or replace)