iMac will not start

You checked the power cord and outlet to make sure that there is power?

If it does not turn on, go to > https://support.apple.com/en-us/HT204267

If it turns on but does not startup, go to > https://support.apple.com/en-us/HT204156

If neither of those get the iMac working again, then you will need to contact Apple Support.

Start here: If your Mac doesn't turn on - Apple Support

Please post back your results.