Phone Xperia Miro doesn't start? Whats wrong?

Hi Folks,

a friend gave me an Xperia Miro. I sould change the glass.
So i tried to turn on before change: nothing happend.
I changed the glass, tried to turn on: Nothing
Xperia on the charger for 2 days: LED blinks steady red (about 1-2s between blinks).
So i thought it was the battery - bought a new one.
Battery loading: Battery gets warm (!?!) but still nothing happens.
Tried hard reset (vol up & power) LED turns blue steady (while connected to Powerplug)
As i plug off, the LED stopps. When trying to turn on: nothing happend.
Another few hours of Charging (always stock cable & loader): After turn on 3x red for 10s. Afterwards red 1x each 1-2s.
Tried to connect to PC & SPCC: No connection - phone not detectet.

Phone was never rooted or similar.

Is there any trick left? Is this a known issue? Warranty is over - he said it worked before screen change but he didn't use it for months.

Thanks,