How do I get Windows 10 to boot after update?

I installed windows 10 on top of windows 8.1 and straight after the update my computer will not boot with any of my windows operating systems, I have :-
windows 7 on one hard drive
windows 10 on top of windows 8.1 on a seperate hard drive
and ubuntu an another hard drive

and after windows 10 updated itself (which took over 7 hours to do) now my computer will not boot using any of the windows installs I can only boot into ubuntu I cannot even boot using my windows 8 DVD I have tried everything I can think of but every time I try I end up in a loop going from POST to windows then it comes up with a message saying it cannot complet and will now reboot to fix things but then carries on going round and round can anyone give me a hint to either boot into any windows I have installed or even reinstall windows 8 from DVD then re install windows 10 again.

regards Poppy Ann.