Hi mate, try to press and hold volume up and power button untill it gets you to boot menu. When you get there try your volume up and volume down button and check that they are working properly. A common issue is that volume down button is stuck and is permanently on and when you try to boot the phone it does permanently force restart. If everything is ok with your buttons, try to select wipe cache and then reboot the device. If problem remains, then if you don't care enough of your data you could try factory reset from the same menu as i mentioned before. Note that the device will boot as new but it will be locked with your gmail account.