No downloading attachments, no downloading apps, not entering any inputs on a field, etc.
I don't welcome any random apps, browser plugins, extensions, etc. from random sources on my PC. It's been well protected by me for years and this is actually the first time I got to this kind of situation. Aside from that, all my keys, passwords, and any personal stuff can't be found on my PC, email, notepad, files, etc. as I stored those on my external flash drive and a backup USB drive.
Sometimes we make sure that all these security requirements are duly followed but we forget that the password needs to be strong enough and not easily guessable. I hope you have kept a stongest password for the account so it cannot be guessed by any hacker.
Also i can understand our situation as how you would be feeling with your account locked with none of your mistakes. I hope things are settled quickly and you get back access of your main account soon.