Since the update of Dec. 9, many of my longtime accounts do not recognize me, and I don't currently have workarounds. I also keep getting nasty messages that I'm in mortal danger because I've lost contact with iCloud. I have a son-in-law and grandchildren with extensive computer expertise who will visiting during the holidays, but during the holidays, but I would much prefer to be partying with them than holding computer repair clinics.

Are there any measures that a complete amateur can take to solve the worst of this Gordian knot?