My Edge 840 won't turn on after the screen froze while charging

Whilst my unit was charging yesterday, it froze at 98% battery charge.

I held the power button for about 10 seconds until the screen went blank. I pressed the power button again, but it didn't turn on.

I tried numerous hard resets by holding the power button for 10 to 60 seconds, but it didn't turn on.

I tried numerous combinations of holding the lap, start/stop, and power buttons for 10 to 60 seconds, not attached and attached to a power source, but it didn't turn on.

I connected the unit to my PC and tried the above methods to turn it on, without success.

Any additional advice?