Administrator privilege has been changed automatically to Standard user

Product information:Windows server 2008 R2 Standard

Scenario:

1. System was running with administrator privilege but after a certain period administrator privilege has been changed automatically to Standard user. Nothing is accepted without administrative privilege.
2. Administrator account shows “unknown account type”.
3. Cannot install, uninstall, run, shut down any third party software. 

Possible Reason:  

This system is running under in unique VLAN and not shared with internal network. But one terminal has full privileged to access the system and that terminal is connected (might be) with internet and personal entertainment.

Action Taken:

1. Tried to change account type from Standard user to administrator.
2. Tried to create new.
3. Tried to access Server Manager
4. Tried to Restart/Shutdown windows

Result:

Failed to run any operation without having administrative privilege.

Help seeking:

Please help me to recover the problem to get back administrative privilege.