Usually in Android mobile phones,when you hold the power button it should display a menu with a set of options which allows you to power off your phone,turn it into airplane mode or restart the phone.But sometimes, this function may stop working.
To fix android phones with this problem, restarting the phone by taking out its battery and re-inserting it may sometimes help. But there are Android phones in which the battery could not be taken out and in such phones,restarting and checking is not a helpful option.
Restoring the phone to factory settings works in this case. To restart your android phone, follow the given path:
Settings -> BackUp& reset -> Reset phone
But before performing this operation, it is a must to back up the data of your phone by the default function present in the phone :
Settings -> Backup&reset -> Backup &Restore
or you can as well backup your data using a third party application.
By this method, your android phone can be fixed.