Try letting down the power button until the screen turns off, then try to turn it on normally.
If your smartphone is locked in the boot off and try again with the following steps:
1- With the smartphone off press and hold the left button "Volume +", press the power button for 4 seconds and release both buttons.
2- Here must appear the menu of recovery but I think it may vary depending on the model, the android if you slept with the red triangle press the power button to display the recovery menu appears.
3- In the recovery menu clears the cache and Dalvik, then restart your smartphone.
4- If your smartphone is locked again at startup switches off and enter the recovery, now try installing the OTA (ie an error to try to repair your firmware), then restart.
If your smartphone is locked again at startup will have to make a factory reset, all your personal files and data will be erased (it would be good to try to rescue whatever can by ADB or via custom recovery Backup).