Well, it may just be that the tablet is 'loose' on that part of the device. Make sure it isn't opened up at all. If that doesn't work, it could be the connection to the display or the display itself. Fixing either would require opening up the device.
If your tablet is under warranty still, then you should contact the manufacturer about the issue. If it isn't under warranty (or it was voided due to a drop, etc.,) then you can either try fixing it yourself or you can take it to a local tech.