Malware on desktop computer?

6 weeks ago a full-page 'warning' took over my computer screen. It read: Your computer is blocked. Fortunately your Micosoft program has protection. Do not shut down computer to prevent loss of

malware identity. Call this number immediately. I did and the voice on the other end didn't sound

credible and I hung up. I shut down computer and have not used before today. Computer works

as usual but I wonder if I need support to help me check for Malware. Computer is an imac desk top

dated later 2013. Never had trouble with it before this. Advice is appreciated.