Follow the directions here;
It's important that when you reactivate your current system, you change your password and DO NOT GIVE IT TO ANYONE.
It said "This ps3 could not be activated Your account already has 2 ps3 systems activated for game content."
As of November 2011 you are only allowed TWO systems on one account.
I can't help but think there's something you're not telling us. Deactivate the systems that you aren't using, and get it down to the one system you are using. Then change your password. Then DO NOT GIVE YOUR PASSWORD TO ANYONE.
This is not because your account got hacked.... the PSN has many security measures in place to prevent stolen accounts. Not even social engineering will do.
If you "got hacked" you knowingly gave someone access to your account
In the future remember do not ever give out your account info to anyone.