I was using my phone when suddenly the touchscreen was dead partially on the left border of the screen. Therefore A, Q, and R key and the above part (along the left border) are unusable and the rest just works fine. Plus I hadn't made any damage on the phone and it hadn't felt off to the ground lately. So the question is how can I know this problem is caused by software issues (i.e, operation system, third-party applications,...) or by hardware issues (i.e, the touchscreen itself...). My Android version is 2.3.3 (upgrade from 2.2). Thank you all.

P.S: one more question, could I downgrade the O.S by hard reset? Just curious :-)