Ok, I think I already know the answer to this one, but it's important I try a bit more.
I have been using a wubi installation of xubuntu for about a month, keeping my windows installation for some data I needed.
It's been working fine until this morning when I got to the usual boot menu, but trying to boot into xubuntu simply sent me to the grub screen. Imagining it was something broken in the mbr or similar I booted into a sysrescue cd that I knew would come in handy and booted into an xfce session. I mounted my windows partition and found that the disk I was expecting to find at /mnt/windows/ubuntu/... was gone. There was simply nothing there.
It's not a pain to re-install ubuntu, and I am now intending to ditch windows permanently (or possibly keep it on a small partition for testing), but I had about 2 days work of solid project on the drive, which I hadn't backed up because I was working on it as a test, and hadn't realised how much there was until it was gone. Is there any way I can get this back? If I knew roughly what data would be on the disk then I could find it and undelete it? I haven't booted into windows or done any IO, so I'm holding out hope and not formatting the disk (yet)