if you can boot into grub one time and then the next you can't, reinstalling grub will not solve this issue as it's not with grub. there's some other underlying issue with your current setup. what is the error? when you say grub hangs, where does it hang? it only hangs when trying to boot to ubuntu but not windows? do you have a multiple drive setup or just 1 hard drive which both os's on it? these are some of the questions that need to be answered. but no matter what you have to think about it, if grub works one time, then it doesn't the next, then it does the next, than that means that all of grubs files are there and it's loading so i don't think the issue is with grub.