Database alert log might show following error.
You might have already ensured that no other instances of this Oracle database is running on this server.
Check for the file pointed by alert log. In my case it was $ORACLE_HOME/dbs/lkBRTEST1 i.e lk
. Delete this file and starup the instance.
In some cases sgadef
.dbf file under $ORACLE_HOME/dbs can also cause the issue. You can delete this file as well and try to startup the instance.
This issue is more likely to happen when ORACLE_HOME & database files resides on NFS. NFS should be configured properly to release any locks after a server or database crash.