Why is my Lg g3 screen stuck on a gray screen?

[DayN9ne]

iv'e had this phone for not too long and well everything is just fine, love the phone but this happened today after i got back from school i was feeling the phone in my pocket, but i didn't know that i was actually touching the phone and when i pulled it out it was stuck on this dark gray screen with a small white/light gray camera icon at the bottom. i don't know what this is, but i can still charge the phone, i can listen to music via pressing the play button on the headphones, if i hold the power button i can feel and hear the option to turn off the phone, but nothing happens i can turn the phone on and off i can also turn on the phone using the knock code so i know my touchscreen still works. please help, thanks in advance apparently i can screenshot it to?