Many people have posted here in the past who had inadvertently locked the screen and didn't know why the device was unresponsive, so it's worth bringing up with every new poster that has a similar problem. Please don't think that we think you're incompetent or anything, it's just worth double-checking that first before looking elsewhere for what's wrong.
If the device is more or less in its stock condition, then it sounds like you have a lemon. Perhaps it's worth checking to see if there is a newer firmware version on the PB website that you can flash to the device. If not, you should probably get a replacement device.