Mi Max stuck looping on Fastboot screen, doesn't start

My phone was working fine, recently it's side buttons got defective, around the same time once when restarted I was treated with 'Fastboot' screen which appears on holding down the power button and then disappears (while trying to turn on the phone).

The side buttons are now replaced and functional but phone can't be switched on, everytime i hold power button to switch on the phone I get the Fastboot screen which appears and disappears. If the power button is held down for long, the Fastboot screen keeps appearing and disappearing again and again and the phone doesn't start. Please help :(