1. Hold down power button for at least 15 seconds until the question mark goes away or it starts to boot.
2. If 1 doesn't work, press and hold the volume down and power button for at least 15 seconds and see what happens.
3. If you can take the back off, do so and remove the battery, put it back and power up and see if that helps.
I'm sure others will make suggestions as well.
It would help if you joined the site so you can reply to other postings. Also, let us know what phone and OS you are running.
Good Luck.