Alertlog: Shutdown - Instance shutdown aborted

Dear AllI have automated database shutdown at night. Yesterday database was not shutdown. When I check alert log found following messages. How can I investigate reason for shutdown failure?

Thu Apr 11 03:25:05 2013
Active call for process 30742 user 'dbmgr' program 'oracle@testapps.niti.edu.sa'
SHUTDOWN: waiting for active calls to complete.
Thu Apr 11 03:34:21 2013
Incremental checkpoint up to RBA [0x73.17846a.0], current log tail at RBA [0x73.17847b.0]

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Thu Apr 11 04:20:08 2013
SHUTDOWN: Active sessions prevent database close operation
Instance shutdown aborted
Thu Apr 11 04:25:08 2013
Starting background process SMCO
Thu Apr 11 04:25:08 2013
SMCO started with pid=14, OS id=26254
Thu Apr 11 04:34:26 2013

Regards,
Salahuddin.<!--[if gte mso 9]><xml> <w:LatentStyles DefLockedState="false" DefUnhideWhenUsed="true" DefSemiHidden="true" DefQFormat="false" DefPriority="99" LatentStyleCount="267"> <w:LsdException Locked="false" Priority="0" SemiHidden="false" UnhideWhenUsed="false" QFormat="true" Name="Normal"/> <w:LsdException Locked="false" Priority="9" SemiHidden="false" UnhideWhenUsed="false" QFormat="true" Name="heading 1"/> <w:LsdException Locked="false" Priority="9" QFormat="true" Name="heading 2"/> <w:LsdException Locked="false" Priority="9" QFormat="true" Name="heading 3"/> <w:LsdException Locked="false" Priority="9" QFormat="true" Name="heading 4"/> <w:LsdException Locked="false" Priority="9" QFormat="true" Name="heading 5"/> <w:LsdException Locked="false" Priority="9" QFormat="true" Name="heading 6"/> <w:LsdException Locked="false" Priority="9" QFormat="true" Name="heading 7"/> <w:LsdException Locked="false" Priority="9" QFormat="true" Name="heading 8"/> <w:LsdException Locked="false" Priority="9" QFormat="true" Name="heading 9"/> <w:LsdException Locked="false" Priority="39" Name="toc 1"/> <w:LsdException Locked="false" Priority="39"