As long as you can boot your Ubuntu live media, you can fix things. As a background task, next time you boot or next get into UEFI settings/BIOS, try to see what your machine's firmware version is, and do check the vendor web site for any updates. Taking a look at OldFred's suggestions, try the efi boot, which you should be able to start with a function key (each machine is different, can't say which one). The correct function key should list the boot devices for you to choose -- if you just see a hard disk (HDD) choose it, and you should then get to choose ubuntu or windows. Try Windows, but after running boot-repair, you will probably get the grub menu, just like you should when you select ubuntu. That's because boot-repair has renamed the windows boot files, and put a copy of shim.efi in their place. You can rename them back yourself, or run boot-repair and "restore backup files". The backup files all have bck in their names, so it's easy to tell what they should be (same name without the bck). So, where are these files you ask? They are in the EFI boot partition, which MAY be automatically mounted by the live media. Such automounts are done in /media, so look for any subdirectories in /media:
Expect a directory with 8 or 9 numbers and letters, look into it:
Expect to fine a directory named EFI. Look into it and you should see directories named "Boot", "Microsoft", and "ubuntu", and may others which the vendor may put there. The Windows files are under Microsoft in another directory named Boot, so
should list all the (renamed and backed up) windows boot files.
You can look into the ubuntu directory, and see the shimx64.efi and grubx64.efi, and grub.cfg files ubuntu needs to boot.
use the -l switch after the ls above to see the sizes of the files -- since they have been renamed, you can tell what they really are by their size, expect many files to be the same size as the shim.efi file -- they're copies. Now, your efi boot menu probably points to /EFI/Microsoft/Boot/bootmgfw.efi (or bootmgr.efi I don't know what they do differently). but both are probably copies of shim. I'd guess the original bootmgfw.efi is now named bckbootmgfw.efi, so just copy it back to it's original name (overriding the shim copy):
OK, now you can try the efi boot menu again and see if you have windows back. If you can run boot-repair, all those renames should be done for you by checking the restore backup files, but now you know what's going on under the hood so to speak.
sudo cp bckbootmgfw.efi bootmfgw.efi
sudo cp bckbootmgr.efi bootmgr.efi
One step at at time, and you'll get there. Your Windows install probably is fine, just the booting was messed up.