Hello
At the time of the reboot was your battery atleast halfway or was icon sitting at red?
•if it was red its unlikely to boot up because a boot up takes a lot out of it.
Steps to solve that are:
Remove the battery and insert the charger and now reinsert the battery. Leave to charge for 3-5 hrs hours...if still no luck then repeat the whole procedure again.
•If the battery was full but now doesn't want to boot:
Try a safe boot by removing the battery and hold down back key and now reinsert battery when the loading is a quarter way release the key........if the phone boots up proceeding with deleting any installed apps then was installed just prior to this issue.
• If that fails then remove battery and hold down "end call button" now while holding down the button replace battery, when red led lights up release the button and press and the "call button " for 3 seconds and release and press "end call button" ...........continue this process and hopefully you see hourglass pop up. Once this occurs leave the buttons alone as it should boot up.
Once it boots up leave on charge for 8 hrs.
If all this fails seek out repair centre.