virtualbox.org

any way to fix these without reinstalling everything?

host win 7 x64
guest win 8.1, win 10 and win 7

guests hanged a few times and now won't even boot

Given that you provide zero diagnostic info, not even program versions, all anyone can do is speculate. However so far it sounds like it may be the same as viewtopic.php?f=6&t=78248.