Restart of unreal hangs at 95%

Everything was working fine, I was editing a blueprint. I clicked to compile blueprint and UE editor hung. I closed it in task manager and now when I re-open it stops loading at 95% - now work has ground to a halt and I have no idea how to get my project back.

A blueprint based on the blueprint library I was editing (a sub class of a subclass of Actor) seems to have been causing the crash. I manually removed it and an editor started, now I have changed the parent blueprint and put the subclass back in (manually) - so why has this happened? I thought initially it might be an infinite loop in the BP? will it happen again? what should I look out for?