HI Pradeep,
What O/S are you running?
1. If windows check that all processes in task manager for Oracle and in MMC was stopped. If linux/aix run command ps -ef |grep ora
2. Are there any processes still running from old, if so check if there are backups or statistics or unfinished jobs running keeping database from shutting down as the others said check your ora alertlog.
3. Once all is checked shut down now processes running ensure you start cleanly. The error could also be a max number of sessions meaning it never shutdown properly.
Please advise on your log.
Kind Regards,
Johan