freeze installation screen el capitan from snow...

Dear friends,

I decided to upgrade my imac 2008 with Snow Leopard 10.6.8 to El capitan. The installation process started but it stucked and the progress bar was at almost the starting point for about 3 hours. I shutted down and the same again. It stucks on the same point. Any ideas at least to have the snow leopard again?

I would appreciate any advice. Thank you very much