Why my Samsung galaxy tab 7.0 GT-P3100 not working?

My tap stopped and appeared this message

"Warning!!

A custom OS can cause critical problems in phone and installed applications.

If you want to download a custom OS, press the volume up key.

Otherwise, press the volume down key to cancel.

Volume up : Continue.

Volume down : Cancel (restart phone)

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Then when i choose press Volume up

Appeared

" Downloading ...

Do not turn off target !! "

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Then turning off and not working by any way ?

Help please