HTC Desire 826 dual SIM not booting

My HTC Desire 826 dual SIM phone is not booting. When charger is connected, red LED just keeps blinking.

Before this it was giving message to enter decryption password though it was never encrypted. What ever password you enter, it used to say password was correct but data got corrupted and advise to go for factory reset and Factory reset was failing with some error.

I gave the phone to HTC authorised service center for repair. They not only failed to resolve the issue but also made it worse as it is totally dead now with the issue as I explained in first paragraph. Tried connecting it to laptop with USB but it's not recognised.

It would be great if anybody helps me with solution.