IMac - switches on but black screen

I went to switch my iMac on this morning and I can hear it firing up but the screen remains black. I can feel the fan at the back so it’s definitely on and has power. Does anyone have any advise or does this sound like there’s a problem with the screen? The iMac is 2 years old.

This couldn’t of happened at a worse time as I am a graphic designer and am in the middle of an urgent job. Arghhh. Apreciate any help.

Helen