Some times dbconsole fails to start if it has crashed unexpected. Mostly this is an agent issue that can not start or can not upload some files. Then you can try to clearstate agent.
export ORACLE_HOME=AGENT_HOME. This is usually the db home if you are using dbconsole.
If you are using Grid Control Agent has its own home.
emctl stop agent.
rm agntstmp.txt lastupld.xml
emctl clearstate agent
emctl start agent
After few minutes issue
emctl status agent
and you must see a similar output :
Last successful upload : 2009-10-22 07:31:47
Total Megabytes of XML files uploaded so far : 4.42
Number of XML files pending upload : 0
Size of XML files pending upload(MB) : 0.00
Available disk space on upload filesystem : 24.94%
And you are ok.