iMac white screen at start up(with apple) waits a few minutes and shuts down

My iMac is an 07? Not sure.

It has been slow so I moved everything to an external hard drive

I was syncing iphone photos and it locked up numerous times.

I manually turned off the power. And now it will not start again.

I get a white screen, chime, and a loading bar with apple logo for a few minutes and then it shuts down again

I am

On snow leopard

Thank you! Be gentile with your tech words- after 7 years I still

Don't fully understand this beast of a machine :)