I am/was dual booting Debian and Win7 but Windows7 SP1 wasn't installing because it couldn't automount Windows (Grub2 installed) so without thinking I opened disk manager and set the Windows partition to active, which deleted the Linux partition and Grub with it.

Now I cannot boot into either, and am dropped into a grub rescue shell.

Why would that delete a partition ?.

If it's merely deleted the partition, testdisk will find it.
If Windoze insisted on reformatting it, you're up for a re-install (Debian)

I like boot-repair it has saved me a lot of work lot of times
Boot-Repair , simple tool to recover access to your Operating Systems.

Easy-to-use (repair in 1 click ! )
Free (GPL open-source license)
Helpful (Boot-Info summary to get help by email or on your favorite forum)
Safe (automatic backups)
Can recover access to Windows (XP, Vista, Seven).
Can recover access to Debian, Ubuntu, Linux Mint...
Can recover access to any OS (Windows, MacOS, Linux..) if your PC contains Debian, Ubuntu or derivative.
Can repair MBR-locked OEM computer boot if the original bootsector has been saved by Clean-Ubiquity
Can repair the boot when you have the "GRUB Recovery" error message
Options to reinstall GRUB2 bootloader easily (OS by default, purge, unhide, kernel options..)
and much more ! (EFI, SATA, filesystem repair...)

here is the link:

http://sourceforge.net/p/boot-repair/home/Home/


I hope it works for you
I downloaded it and burnt the .iso to a CD and just boot from it and it has always worked for me nd finds my many partitions and OS'es It has found and enabled bootup of OpenSUSE, Debian, Ububtu, Fedora and Windows, for me so far and.
Back when I was using Ubuntu Maverick 10.10 it came in a package and that's when I discovered it, I tried it for the first time and it worked great, and it will re-install grub legacy or grub2 for you etc.

seems like a haruko mistake. i would boot a live-usb with gparted and confirm the partitions are there and mountable.