Patricia. I had exactly this issue with mine (even to the extent of having another one I could use to compare power supply etc - how weird is that?)
There's precious little you can do inside the recorder itself as the components are ll tiny surface mount and diagnostics without specialised kit are nigh on impossible.
You've proven the power supply, which is good.
If you want to go for it there's an outfit who offer fixed price repair for the Hummy box at £69.95, including return shipping.
I used them myself few months back. Slick operation and rapid turnaround.
Use Royal Mail small parcel (watch the height of the box) and postage is surprisingly cheap - I think Royal Mail are still doing free pickup of prepaid parcels too!
So long as the hard disk itself is OK you should (I did say *should*) get the PVR back with your recordings still in place.
This is the outfit:
One thing: we had frequent issues with our box saying "no recordings" when turned on. We'd do a power off (unplug) and restart and all would be fine for a while. Since the no-power repair the problem has happened much less - ring any bells?