then i guess something is wrong with your windows install ....
try locate it using linuxtools or use windows boot cd (winpe) or reinstall
whatever - it is outside this forums scope

Ok. I'll reinstall Windows but if i do that, GRUB will be removed right? then after reinstalling windows, will i be able to boot in Ubuntu? Or do i need to install Ubuntu too :(

the no sweat way is install windows then ubuntu

if you want a challenge you can try installing grub just from your ubuntu bootdisk - the way to do this has been covered many times in different places - i propose you google some

good luck