Imac won't start. Looks like loading at start

I got my imac about 4 months ago. Today I wanted to turn it on and after the chime I see the apple logo and the spinning wheel and the loading bar. I thought it was updating itself but after the loading bar got full the imac shut down. I had this multiple times and it just won't start. I unplugged the cord waited 10 minutes and tried again. Same thing. It shuts down after the loading bar gets full.

Since I am new to this I can not tell which operating system it is on but I got an upgrade about 2 weeks ago.

Any help is much appreciated!