Hello,
As far as I understand after understanding your topology, you should **HAVE** the routes in routing table as it was working fine. Are there any chances of configuration not written i.e. changes were made and not written in startup config?.
I am not sure whether there are some static routes in CORE router 7200; If that is the case, you may consider using commands:
redistribute connected subnets
redistribute static subnets
Please provide the configuration of core and 3600 router (after removing password and SNMP string).
Also verify the output of "show if OSPF database" for sequence number as reboot cause sequence no algorithm to be started again..
Hope this will help you in troubleshooting.
Thanks,
Deepak