well lets see!
1.Do you have the SA Account name/password. If you do there is your answer. If you don't see 2
2.Do you have another account with system administrator permission?
3.Are you using windows authentication?
if you the sa account you can access that user account to make changes to any account in your database
Using windows authentication, your domain account (if it has the permissions) already logs you in authentication.
if you are using SQL authentication, using the sa account or any other account with privileges. go to security, logins, right click user, under password type in new password then verify the new password. I hope this helps.