27inch iMac screen won't turn on

hi all,

my imac is about 3months out of the box and a few days ago i went to turn it on and....nothing. i could hear the start-up chime and the mac clicking and making "doing" noises, also i could clearly hear the volume toggle up and down. That said, the screen remained, well, dead and just black.

So, the first thing on my agenda for the weekend was to take my imac into the shop. As i was organising the equipment, i tried one more time to turn it on.

It worked.

For me, that's a bad thing as my mac may do this again. It was out of action for 3 days now it's back. So now i am on here.

Does anyone know why this would happen, or has experienced a similar issue and if so what the solution to fixing this would be.

I'll take it into the shop, buti am wondering if there is a simple reason this has happened

Thanks in advance and cheers!