When your linux root partition gets suddenly deleted and you are stuck up with black screen with prompt
grub>
There is no need to reinstall windows. You can boot the windows from this prompt
Assuming windows is installed on first hard disk of your system & in the first partition the type the following commands sequentially & you can get back to your windows
grub> root noverify (hd0,0) grub> chainloader +1 grub> boot
If windows partition is on 2nd partition of the first hard disk you have to use (hd0,1)
if windows is installed on the first partition of the second hard disk then use (hd1,0)