Cant login to windows server 2008, it says user or password incorrect, dhcp also not working

Today I started our Windows Server 2008 Standard SP2 Domain Controller PC, it was loading very slow in start and after loading it was not accepting its Active Directory Administrator Password, even I tried to login with another users' password but still it was saying "Username or Password is incorrect", The DHCP is also not giving IP to clients but manual IP settings makes the share drives work properly.
I'm sure the Username and Passwords I'm typing are correct but the problem is in Windows.
please help me with this issue

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