Dell Inspiron 15 3558, changed boot mode to Legacy and back again, now it can't find the OS

As per the title, I changed this laptop's boot mode from "UEFI - Secure Boot" to "Legacy" in order to do a boot-time virus scan from USB (with ESET - it didn't find anything fwiw).

However, when I changed boot mode back to "UEFI - Secure Boot", it says it can no longer find any OS.

When I use a Win10 disc to boot into rescue mode, if I go into the command prompt and use dir, I can see that all the files are still there, so the drive hasn't been wiped (thank god) or anything.

I've tried using Win10's default "Startup Repair" option, which doesn't appear to work. I've tried using bootrec /fixmbr and /fixboot, and even /rebuildbcd, none of which are helping it find the OS.

Any thoughts?