Help with diagnostics or...

Newbie to W10.
Do not know how to explain in detail.
A screen pop up a few days ago instructing me to call a number that my computer was corrupted.
A few minutes later it crashed. All that happened after rebooting was 'your PC need repair. Install recovery' or something like this. Recovery was successful but now the computer is slower and does things it did not do before.
Example, 123D crashes suddenly, other programs do also and other software problems that did not exist before.
AVG antivirus finds nothing wrong.

How does one diagnose W10 to find problems?

dlw

Welcome to the forum the first message was a virus that had infected you the question is how you did the restore we need to know as you may still be infected and it could be Ransome were that could spread to any other PCs on the network

Download a disk from Microsoft. Did the repair that way. Had a recovery backup of my own.