If you provide a brand and model number this would help. Also, this may come across condescending to some, but did you touch the "lock" and slide your finger all the way to the right while dragging the lock in the process? I know that may sound laughable to some, but I've ran into that problem with my mother when she bought a Droid 2. Had to teach her that little tidbit amongst many other lessons.
Do some tablets have different unlock methods than that of Android OS phones? Never messed with one.