I turn the board on by connecting into Vin input 5V fron 7805 from another board.
This does nothing for trying to see if the onboard regulator is functional, and does not answer my questions:
(#1)what is the voltage at the 5v pin?
Now, if you do not have a 2.1mm plug that's a different issue, but (#2) what are you powering the 7805 with, and (#3) what is the measured voltage output of the 7805?
5V from 7805 from another board to 5V input
If everything is connected properly and operating normally, this shouldn't have caused a problem. You can feed 5v (from a well-regulated source) directly to the 5volt rail, I do it all the time.
Do you have any idea fixing it?
Google does... "Arduino DFU mode"