You may have either,
A. Typed it wrong.
B. Forgot it.
C. Gave your password to a friend to account share.
I highly doubt it was hacked, you probably fall into one of the three categories.
If your Login Server account is tied to your forum account, you can go to User CP, click on Login Server.
Steps in order.(Color Coded)
1. Click on UserCP
2. Click on Login Server Accounts
3. Click on Change a Loginserver Password
