I have seen this problem three times now.
First one was resolved by a gentle rap to the case.
The second one resolved by power off.
Third by removing the battery.
Is it possible one of the apps is causing the problem ?
Could it be a OS issue? Hardware issue?
I've installed labyrinth lite, Anglemeter, seismometer and pedometer widget.

vhb_q8, did you begin having problems after installing one of these?