I suggest you to install an app providing an on-screen equivalent of the hardware navigation buttons: this will allow you to use your phone and set up backup or export your data. Then you'll be able to try everything (including a factory reset or a trip to the service center) without risking your data anymore.
Google pointed me to https://play.google.com/store/apps/details?id=tw.com.daxia.virtualsoftkeys
If I may, always backup the data you care about. ;)