Pop!_OS installer would normally just add its files to ESP (EFI System Partition). You may have accidentally chosen to format it during installation or it was damaged when resizing failed. The standard course of action in this situation is to restore from backups. Let this be a lesson: make regular backups of entire system (partition images for non-data drives) and make additional backups immediately before messing with partitions. And if something goes wrong in the process, try to understand the problem and fix it instead of continuing!
The repair is possible withoutThe repair is possible without Windows ISO You can find most needed files in the Windows ISOfolder, but you'll have to generate boot configuration store for your specific system. You'll need some kind of Windows recovery environment to do this, preferably ISO of the same Windows version which you have installed. Older builds of Windows 10 didn't use hybrid 32/64-bit ISOs, so if your bandwidth is limited you could consider downloading them (if you can find trusted sources, because Microsoft doesn't provide them anymore).
Boot files are located in C:\Windows\Boot\EFI. You have to copy them to <ESP>\EFI\Microsoft\Boot. You may also have to add the bootloader file to UEFI boot list. You can do this in UEFI setup. The bootloader binary is called bootmgfw.efi.
Boot configuration store can be generated by executing bootrec /rebuildbcd in the <ESP>\EFI\Microsoft\Boot directory.
I've used this website to refresh my memory. I've done this repair in the past and I can confirm this method worked.