Maybe the hacker hacked my email, and my bitcointalk account password is same as that of my email account(I have changed them different passwords and applied F2A in my email account.).
You should NEVER reuse your passwords for anything again, and I hope you learned a lesson about that.
Use some password manager like KeyPass for storing and generating new strong password for each website.
Maybe the hacker instructed Trojan virus in my computer, because I had found some virus when I used windows defender to scan my computer some months ago.
It could be a false positive, but it's possible that you have some clipboard malware that could collect your passwords and any keys related with crypto.
Switching to Linux may also be a good idea so you won't need to use any antiviruses, and offline computer is much better for dealing with crypto.
Good ideas. I will try to do that now.