Does it respond at all when there is no lens mounted? Since you are saying it’s totally not responding, it is less likely (assuming none of the buttons do anything and cannot get into the menu).
You could try leaving the camera without the battery in it for several days (but might need weeks, not sure on the life of it), to discharge the clock/setting internal battery, then plugging in a fully charged battery.
Could check to see if the mirror is moving correctly (I had it once, where the mirror stuck half way when the camera lost power, and didn’t reset correctly, but while I had Err, it still turned off and on)
If possible, try another battery, as it could be the battery is defective and not letting the camera power up either. If you don’t have one, a local camera store may have one you can use there to let you see if that is the case.