Hi
What is the model number of the desktop?
Are you trying to boot from a USB recovery drive?
Try enabling Legacy USB (or CSM - not sure which setting is applicable as you didn't give a model number) and then check if you can select the USB as the 1st boot option save the changes , insert the USB and restart the desktop.
If it boots from the USB recovery drive, go to Troubleshoot > Advanced > Startup repair and check if you can restore the startup so that it boots from the HDD.
If you have Win 10 installed, a recovery USB can be created from any known working Win 10 computer. In the host PC go to Control panel > Recovery > create a recovery drive and follow the prompts. You will need a 8GB USB stick and about 40-60 minutes of time to create the recovery USB.