The described symptoms indicate that the tablet may have gone into APX mode for some reason. If this happens automatically with normal use, it may be caused hardware failure.
Start by just forcing the tablet to power down by holding the power button for at least 5 seconds. You should not feel a vibration when it powers off using this method. Then just press the power button for 3 seconds to power back on. If the same problem occurs, then you may also want to try Method 2 of: [HOW TO] Perform A Factory Reset.
Follow the instructions carefully. It can take few tries to get it right.
If that does not work, you may have some corruption in one or more partitions preventing normal recovery, and this may be caused by a hardware fault. In that case, there are ways to attempt repair/replacement of data on affected partitions using a PC and the USB cable supplied with the tablet. A number of such recovery methods are discussed on this site and at xda-developers.com.
If the easier resets noted above in the second paragraph don't work though, then it seems possible that hardware has failed, and that no soft recovery method will work. Unless you got into this while trying to install a custom ROM, or by some unfortunate mistake while using root privilege, then it seems more likely to have been caused by a hardware problem than a software problem.