i take it you did not read nor heed any of the warnings about upgrading windows
the very BEST possible of the bad situation is windows replaced grub as the bootloader
it can not see read nor boot RHEL partitions and can not boot RHEL
use the RHEL 6.5 install dvd and reinstall grub
( see the redhat install guide on the redhat documentation page
https://access.redhat.com/site/docum...erprise_Linux/
or search the redhat knowlage base
https://access.redhat.com/search/bro...#?&language=en
login with your redhat credentials from the REQUIRED support contract you DID buy
-- the support contract IS REQUIRED !!! and you need to buy a new one EVERY YEAR
and reinstall GRUB
or
the WORST thing happened
the 8 upgrade REPLACED ALL OF RHEL
and there is no longer a redhat install on the machine
( i hope you do have a back up of any needed data on the redhat partitions )
windows can overwrite a linux install and replace it win windows
there is a reason that Microsoft Windows is often compared the the star-trek "Borg"