Press and hold the power button to force a shutdown, then press the volume down key when it restarts to access the bootloader. Then you can get into recovery, and do whatever is necessary to get it going again - what I'd try first would depend on why it's in the bootloop, but erase cache+dalvik, factory reset, flash a new ROM, restore a nandroid are things you can try.
You might want to just force the shutdown then see whether it will start normally, just in case.