Touchscreen not working

My iPhone just stopped working properly today - the bottom line of the touchscreen doesn't work, I mean, it won't click anything. It didn't fall or get wet or anything in the last few days.

To be more specific, the bottom line, the one where you find the SMS text input and the SEND button - or when you are writing, where you find the space bar. To be even more specific, if this were the iPhone screen:

OOOOO
OOOOO
OOOOO
OOOOO
OOOOO
OOOOO

The last line isn't working. I've tried resetting and everything but it won't work, and without it I can't write anywhere.