Canny Lass,
I've just done a Win10 upgrade without any problems. Like you for this site my login details are remembered. I didn't have to login to get back into this site.
Do you have any security software running that may be being extra protective since the upgrade?
You need to speak with someone who knows about these things.
It may be worth a google to see if anyone else has had similar problems. Search " passwords not saved since Win10 upgrade" or similar. Or, "a popup has appeared since Win10 upgrade" You may get lucky.