I have a Lenovo Ideapad Flex 10 that dual boots with Windows 10 in UEFI mode and Ubuntu in legacy mode.
On boot up I get the GRUB rescue screen, but fortunately I found that by pressing Fn+12 I get a different and basic menu, but I can only get into Windows.
When the problem occurred I deleted all Ubuntu related partitions. When pressing the Fn+12 buttons, I get the name of the hard drive which when selected just takes me to the same GRUB rescue prompt which allows me to get back into Windows 10.
The third option is Ubuntu, which gives me a GRUB prompt (Note 'GRUB' not 'GRUB Rescue'). Now that I have the flexibility of getting into Windows, is there something I can do to rectify the problem? USB ports don't work until I get into Windows so I can't restore its original bootloader.