How the bitcoin forum account can be hack 
Are you asking how it's possible your account was hacked, or are you asking what to do once it's hacked?
How it' possible:
- The forum database was compromised many years ago, if you haven't changed your password since then, a hacker might have brute-forced your password hash
- You might have visited a phising site, and entered your credentials unknowinly giving them to hackers
- You might have used the same username/password everywhere, and some other site where you used the same credentials might have been hacked
- Your device might have been infected with a keylogger or a virus
- If you saved your password somewhere in the cloud, this cloud provider might have been hacked
- You could have clicked "save password" in your browser when logging into bitcointalk on a public computer
- You could have told somebody your password, maybe you were a victim of social engineering?
What do do:
Can you sign a message with an old, staked address? If not, there isn't much you can do...
