Samsung Galaxy Tab P-1000 no longer recognised by the computer
Try starting your Galaxy Tab in Download mode.  This is done by completely shutting it off, and then holding both Power and Volume Down buttons together until a big yellow sign with a droid holding a shovel appears, with "Downloading, do not turn off target" message underneath.  If so, you can try flashing the stock ROM via Odin or Heimdal utilities.

You can find the detailed instructions on the Development section of XDA-Developers forum.