Not able to connect to database with Application userid.

Hi

I am getting beloww error very frequently in database log and not able to connect to the database.

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Fatal NI connect error 12170.

VERSION INFORMATION:
TNS for 64-bit Windows: Version 11.2.0.2.0 - Production
Oracle Bequeath NT Protocol Adapter for 64-bit Windows: Version 11.2.0.2.0 - Production
Windows NT TCP/IP NT Protocol Adapter for 64-bit Windows: Version 11.2.0.2.0 - Production
Time: 01-FEB-2012 18:00:34
Tracing not turned on.
Tns error struct:
ns main err code: 12535

TNS-12535: TNS:operation timed out
ns secondary err code: 12606
nt main err code: 0
nt secondary err code: 0
nt OS err code: 0
Client address: (ADDRESS=(PROTOCOL=tcp)(HOST=10.xxx.yy.xx)(PORT=65503))