That's might odd! Ok couple of things... do you have a case on the phone as sometimes these interfere with the side buttons which in turn disables the hardware keys on front.
If you don't then I think you will have to re-flash the firmware using either PC Companion Support section or Sony Ericsson Update Service. This will wipe your phone back to factory settings so make sure you have backed up your contacts to your SD card or sync them with Google or SE sync. Apps will also be removed but you can use Appbrain online and download the app on to your phone and then sync them. It will then keep a list of all your downloaded apps to reinstall afterwards. You will have to set up your email again too. BUT, this should hopefully resolve the issue if it is a software glitch that is causing it.
If a reset doesn't sort it, then maybe it will need looking at by a technician so would suggest you contact your carrier / retailer as a starting point.
Let me know how you get on.
