Hi MarcusW,
you could perhaps try to do a BIOS recovery - if you know which BIOS you had in your NUC before this happened, you can download the corresponding .BIO file from the Intel NUC download center, save the file to an USB flash drive and try to recover the BIOS that way (it should switch all settings to default). I think there was a more detailed procedure for that (i.e. BIOS recovery) on the Intel support web.
Since you say you disabled legacy boot, and if you originally installed your OS with UEFI option, maybe there is the problem - if so, I think you will have to recover the BIOS to fix that (enable legacy boot again).
If you get an error message "bootable device has not been detected", it is also possible something happened either with your (Windows?) installation - you can attempt to fix it with Windows recovery disk (again you will need an USB flash drive and another PC where you can make the recovery disk).
Afaik, by default the NUC boots first from the SSD, then from USB (but I might be wrong on that one...) - that could explain why you are not able to boot off the USB device (since you also disabled legacy boot).
Let us know how it goes.
EDIT: You could also try this post: D54250WYK seems to be bricked
Regards,
/HarveyV