IMac won’t boot, displays circle with a line through it.

My iMac started running extremely slow and when I rebooted it, it made the startup noise, then immediately displayed a white circle with a diagonal line through it on a black screen. I shut it down and unplugged it for 10 seconds with the same result. Any ideas?

FYI, about 5 years ago I replaced the HD with a SSD drive and maxed out the memory, which has worked great.