Lot's of good advice here, unique passwords and enabling MFA where possible being two of the most important. I would also consider changing the password for your email account and checking that you have MFA enabled there. If you want a thorough check of your PC I would suggest using the Kaspersky Recovery Disk,
https://www.kaspersky.com/downloads/free-rescue-disk it will create a bootable USB and allow you to fully scan your PC without interference from something like a rootkit which could mask malware.
There's many ways in which someone could have compromised your account, even a simple password reset if they've compromised your email account, once in it's trivial to delete the change notifications. Or they've simply used a spray attack utilising your email address with a range of pre-generated passwords gained from a previous breach, with something like this is trivial to make simple tweaks to known passwords mixing in different characters.