Walkera devo 10 seems to be dead

It is possible that your power switch failed. I've seen this a few times. The switch used by Walkera is junk.
It can usually be repaired by removing it and then squeezing the tabs gently with a pair of pliers to push the backing board against the contacts. Or just replace it with another slide switch.

It is also possible that the power circuit failed. I've seen this too (though never on a Devo10). If so, it is harder to fix, as you'll need to identify which Tx is blown and replace it.

Unplug the transmitter module from the board and see if anything changes.
Most likely you've blown one of the switching transistors in the power circuit. figuring out which one would require some electronics expertise as you need to know what you are looking for. Fixing it would require some SMT soldiering skills.

I've documented the circuit for the Devo8, but the Devo10 one is somewhat different and I never spent any time on it, so I can't really tell you where to begin.

Sending back the tx wouldn't be economical anyway, so buying a new one cheap is all you can do...

Where are you located?
I would give you better than 80% odds that I can fix it for you. But if you aren't in the US, I'm not sure it is worth it. If you are interested, PM me offline and we can discuss it further

Since it seems unlikely I can help you directly, this is about the best I can do:

Here is the schematic for the Devo8 power:
static.rcgroups.net/forums/attachments/3...08-18%2022.16.16.jpg
The Devo10 should be very similar.
The most likely issue is that you blew out the 8550 or 8050 transistors.

This happened to my Devo8. I dropped in whatever I had lying around the house and it has worked fine since. If the Vgs is different than the original, your voltage measurement on the Tx will be off slightly.

If it is helpful, here are front and back images of the devo10:
static.rcgroups.net/forums/attachments/2...8-devo10%20board.jpg
static.rcgroups.net/forums/attachments/2...5455724-132-burn.jpg
There are higher resolution images out there, but these are the ones I could find quickly.

As I've said repeatedly, I think Walkera's quality control is low. some of their circuit designs are marginal, and the components they used are pushed too close to the operating threshold. We've seen quite a few failures on every Tx model of different sorts.

I don't personally recommend running Li batteries in Walkera transmitters (except the Devo12) without a proper voltage regulator, since the power circuit seems to be marginal, and quite sensitive to over-voltage. Lots of folks ignore me without issue of course.