Originally Posted by ansh25
You can also try holding the Volume down and power buttons for a long period (10-15 seconds), that should normally take you to recovery mode.
That is incorrect. Holding volume up
and power, then releasing power when you see the boot logo, takes you to recovery. Doing the same with volume down takes you to fastboot.
To answer the initial question, hold all 3 physical buttons until the screen or the LED turns off, at which point you can power it back on normally.
The LED is a good tell if it's frozen and the screen is off, like if it froze while charging, because the light will stay on after unplugging it.
LG G2 - Stock ATT 4.4.2
Atrix HD - Stock ATT 4.1.1