If the battery isn't removable, the best thing you can do is leave it alone until it fully drains, then try plugging it in again. If you can, make sure the power is right by measuring the output with a multimeter, or just test the charger and outlet with another device.
After a full battery drain, if you connect it to the power source and it still can't power up, then there isn't much you can do, you'd have to open it to try and find out what happened.