W7 HDrive lost ?

present setup ...
2 internal hd's ... 1 = XP , still runs ok ...2 = W7 was ok for 5 years , then began sometimes startup problems...
then slowed down /froze if on for more than an hour... { see my previous post in this W7 forum }
2 days ago , after i booted into XP , i got a Windows message about the W7 hd needed to be checked for consistency , and an auto CHKDSK was run to verify files and indexes , many files were involved and shuffled around, some orphans reconnected etc.etc...
when CHKDSK finished ,the boot into XP continued on ok ..
BUT , now XP does not see the W7 drive at all ..disk management does not see W7 drive ,
it's as if the 2nd drive does not exist , but is in the BIOS CMOS as channel 3 and can be selected as the first boot etc. etc.
? "if" perhaps there is still data on the W7 drive , and ,"maybe" this is a FAT or MBR , or similar problem , how can i restore the W7 drive , perhaps running an app on the XP drive , or a dvd booting up via CDROM { which have tested and my BIOS motherboard system permits } ..
related question .... i have an external hd onto which i have copied / backed up the W7 drive 2 months ago ...
i don't think it has all the mbr on it ... it won't boot altho i can see it has all the Windows / Apps/ data ..
is there a way to set the external drive to boot ? is there a way to restore the internal W7 so that ii "exists" and i could copy stuff from the external W7 to the " missing" internal W7 ...
all responses will be appreciated...
thanks