I updated to windows 10 a week ago. For class purposes, I needed to install ubuntu. I installed ubuntu 14.04, but then had to delete it for some reason. I deletrd the Linux partition feom windows 10(on fast boot setting ), then when I try to restart the windows it directly takes me to grub command line. As I read in other posts that it might be a problem with windows mbr being replaced, and for that I will have to usr a windows recovery disk. But the problem is, due to fast boot setting in windows 10, I am not able to get the startup optins, therefore I am not able to boot from my USB disk. Any suggestions would be life saving.
Try holding down the F2 Key while powering on, this should take you to your bios where you can disable the fast boot and allow you to boot from your live usb. If F2 does not work why not try all the F keys one at a time to find your system default haha but in most cases it is F2
Hope this was helpful.