If you run into following error in event log. 
ORA-12520: TNS:listener could not find available handler for requested type of server
ORA-12519, TNS:no appropriate service handler found
You should:
I suggest that you read Metalink note 424566.995, which was posted by a user with a similar problem.˙˙He claims that
investigation showed that disconnections from Oracle were not being passed immediately to the listener.˙˙This caused the
listener to (incorrectly) think that Oracle had reached its session limit.˙˙He fixed the problem by increasing the value of
the "processes" startup parameter.˙˙(He says that increasing the value of the "sessions" parameter also worked.)˙˙T
hat doesn't prevent the listener from having an incorrect count of sessions, but it does get around the ORA-12519 error

To do this:
1. Using TOAD connect to DB server with DBA user.
2. Run query ALTER SYSTEM SET PROCESSES = 500 SCOPE = SPFILE
3. Check that settings applies.
select * from v$parameter where upper(name) = 'PROCESSES'
4. Restart Oracle instance.
5. Use select * from v$session to view sessions


D:\oracle10\bin\oradim.exe -startup  -sid orcl -usrpwd * -log oradim.log -nocheck 0 
D:\oracle10\bin\oradim.exe -shutdown -sid orcl -usrpwd * -shutmode immediate -log oradim.log