Comment 18 for bug 610898

@Svante,
You shouldn't have replaced wubildr - that wasn't the issue and doing so likely caused your new problem. I don't know where you got it from and therefore no way of knowing whether that's the reason it won't boot.

When you say "grub is lost" I have no idea what that means. It really helps to just state what happens.

The whole wubildr thing is a bit of an undocumented mystery. A recent change (last year or so) saw the ability of grub to recreate the wubildr (through a lupin override) but this only works on Wubi installs to the same partition as windows. As mentioned in this bug report, this change also prevents the problems you had. I've seen nothing to indicate there are any immediate plans to address this.

The solution to your problems depends largely on whether you have anything important on the wubi install. If not, just reinstall or do a direct install to partition. Otherwise, either pull off your important data (loop mount the root.disk). Finally, if you've got some fully customized install you don't want to lose, it's possible to reinstall wubi, and then just replace the new root.disk with your one. (Make sure you copy it out of the \ubuntu folder before uninstalling). That usually will boot the old install fine.