iMac 201 continually restarts after upgrading to High Sierra

My 2010 iMac, with 12gb Ram is continually restarting after I upgraded to High Sierra. At times it will emit very loud beeps and then shut down, other times there is no warning beep, it just shuts down and restarts. It does this while in sleep mode and at times after it has been sleeping for 5-6 hours with the warning beep loud enough to awaken me in the other end of the house and it will do it when not in sleep mode but not in use either.