PHPBB admin login

[beisan-net]

After I installed my PHPBB forum it was OK ,I managed it modified it .But when I logged out and logged in again I couldn't access my admin area .
Now I can access the forum as a user but not as an admin.
It keeps telling me my username or my password are wrong but I checked them many times and I changed the password but the problem stills.
I have been created 3 PHPBB forums till now because of this problem.
Edit:
this is the error message :

General Error
SQL ERROR [ mysql4 ]

Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (11) [2002]

An sql error occurred while fetching this page. Please contact an administrator if this problem persists.
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