Why is my screen black with the led light blinking yellow

[AC Question]

Hi OMG I'm having a panic attack bc this is my fourth phone in 2 years - I plugged it in to charge and went to have breakfast; came back up to my room and the screen is black. The LED notification thing is blinking yellow because of Snapchat I think, and when I try the power button and stuff nothing works, still a black screen.
Some help would be seriously greatly appreciated!

 

[B. Diddy]

Welcome to Android Central! Which phone? Try forcing a shutdown by pressing and holding Power for 10-15 seconds (if it's a Samsung phone, press and hold Power and Vol Down for 10-15 seconds). Then power on again normally. Does it turn on?