If it is a UEFI PC, you may be able to go into the setup for the PC (UEFI/”BIOS” settings, not the OS) and change to use the Windows boot manager instead of anything Linux-ish, like Ubuntu. When installing linux onto an external drive, it’s usually best to tell the installer to put grub onto that same drive. You can probably use it to install grub on your hard drive, but that’s involved enough that you should exhaust the above suggestions first, and it’s not guaranteed to find your original win7 boot loader. I don’t know how you could use the mint live/install usb to re-image your disk. Or, if you have a win7 install usb/dvd, try booting that, selecting the “repair” option, and see if it can fix the boot problem. Or, maybe you created a rescue partition on your hard drive when you installed macrium and you’ll find that in your bios boot menu. If so, you can boot from that and re-image your disk from your backup media. Hopefully, when you installed macrium you created a rescue usb or dvd. What do you see when you try to boot? What do you see when you use a function key (f12?) at boot time to get into your bios/uefi boot menu? Is that how you are booting from the mint install usb?
You don’t provide enough info to give you anything more than hand-wavy advice.
