Computer has blinking? folder and won't boot up

Hi I'm new here!

I have an iMac with 10.11 OS-X.

Earlier today I was on it with no problems. I came back this afternoon and when I woke it up, the dock was not there and my mouse wouldn't let me select the apple or anything else on menu or open up any of my files on my desktop.

I was hoping it was mouse related, but still same them with new batteries.

I shut it down and when it came back up I had the flashing ? folder icon.

I've doing some research but nothing that was helpful.

Any suggestions would be appreciated!