I have 2 suggestions:
1. Try to switch off your tab and then try to flash an official ROM ( Hold POWER BUTTON for nearly 10 sec to Switch off your tab)
2. Are you able to go to Download mode?
>If yes, then Download an official firmware from Samsung, your tab drivers or simply install KIES and also Download ODIN v1.85
>Go to download mode ( Pressing POWER + VOLUME DOWN button simultaneously till you see the Samsung logo)
>Run Odin and then Connect your Tab to PC and flash the firmware by using ODIN