Hey - You can try the following in their order
- Test a different battery if you can - or simply remove and reinsert the current battery
- Connect it to a computer using its USB cable and then shot press it power key to see it is detected (either by a USB balloon message or sound from the computer).
- Try accessing its 'Recovery Mode' - let us know the AG Mobile device model you have and we will advise on how you can access the 'Recovery Mode'
- Lastly, it may be time to turn in in for service, unless if you do not mind flashing ROMs.