Did you check the battery? Most often, after years of little or no use, it will lose its ability to hold a charge. So, I would start there.
If you have a way to check the charge on the battery, that will help to see if it is dead. Although batteries are relatively inexpensive and easy to find, so if you don’t have any testing equipment, just try a new battery.
If that is not the issue, there are fuses that are related to the charging system that are put in place to stop a power surge from destroying your gamepad. It could be that one of them is blown. They are extremely small and it would require someone that can micro-solder to replace it. I wouldn’t recommend putting a solder blob in its place, although that may sound like an easy solution, it will not protect the circuit, which is why they are there to begin with. Look for the F1 and F2 fuses on the motherboard, but be careful not to poke around them too much, like I said, they are very tiny and easy to knock off the board. If one or both are bad, have them replaced.
Hope that helps.