There is a small chance that a new flash of the boot.img can help.
If the phone has been flashed with wrong img (usualy falty root/recovery procedure), it might help.
Nothing in the SD slot? (if it is, remove it)
Try leaving it connected to a charger for like 2-3 hours.
Connect to charger, hold power and volume up. Then let it be after the 3 vibrations.