Hi

Last night I left my PC for about an hour and when I came back the fan had switched off and the pc appeared to have shut down (monitor was saying no live PC connection) but the power light was still on at the front.

It switched the PC off at the main electric supply but now it won't switch on again.

When the power switch is pressed the light doesn't come on.

It is receiving power though as a small fan starts but the main big fan does one rotation and then stops.

Any ideas? I wondered if the fan was broken and the PC was recognising this and preventing it starting.

It's 3 month's old.

Can anyone offer any advice?

Welcome to Techspot!

It is always more helpful if you take the time and fill in you hardware Specs in your Profile .Answers will be more accurate and faster.
Since yur machine is under warranty I recommend you take it in immediately.
Any digging around you do might ruin your coverage.Right now it sounds like you have some hardware failure.

The power supply has a crowbar protection circuit that locks it off if it shorts out. It won't reset untill you pull out the powercord for a few seconds and plug it back in, that's a possibility (and if I recall correctly the time I shorted out the 12V to the 5V, it acts just like that). if it has CPU overheat protection it can prevent the MB from powering up. Check that the heatsink is still attached to the CPU socket. but as Liquidlen said "if it's under warranty...."