Severity 14 error state 16 on our SQL 2005 ,login failed for user 'username'

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SQL-Server-2005

IIS6

C#2.0

C#

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We had the problem with
source: logon error:18456,severity:14, state 16
login failed for user 'user name' client [XXXX]
XXXX: is our IP address
on our SQL 2005 .

the important points are
1.The error came and gone by itself
2.that to for only one database
3.mainly occurs in night times (7pm-8am)
4.above error is very very less in day time (8am-6pm)

Thanks in advance