My iMac won't start up, Help!

Unplugged the iMac earlier on when cleaning, just plugged it back in to start some work and it won't start up.

When pressing the button on the back the start up chime sounds and a loading screen is shown with the apple logo and a loading bar.. It loads a little and then turns off again.

Keeps doing this and won't progress onto anything..

Can anyone help and tell me what I need to do it get it up and running again :)

Thanks in advance

Milly