Actually I was facing this problem on my testing server on which we've many DBAs. It seems someone changed the passwords of DBSNMP, SYSMAN, and SYS.

Anyway, I have changed the passwords of all of those users and have secured them. After that I refreshed the DB Console configuration files as follows:
1. emca -deconfig dbcontrol db (to drop old config files)
2. emca -config dbcontrol db (to create new config files)

Now its working fine.

Thanks a lot for your time and concern.

--Khan.