Safari Virus
I'm new to this Forum and joined just to thank Bobtomay and OSXX for their help. Between the two of them, I was able to fix my problem. At first, I panicked and thought my Mac had a virus but, when I calmed down, I realized it was just Safari. I couldn't do anything - the pop-up said 'Smart Online Scan', to call a number - and to pay. Also, a warning not to turn your computer off or it will crash. The tab read 'PC-Critical-Error Report.com'.
I want to thank Bobtomay for explaining what was causing it and thank OSXX for 'Do not have Safari set to open last page viewed'. Since I didn't know how to do that, I turned my Wifi off, clicked on reset Safari and unchecked all but 'Close all Safari windows'.
Perfect! I'm so happy I found this forum and thank you!