The screen of a previously working computer repeatedly displays an OS Not Found error message...

Comprehensive and Detailed Explanation From Exact Extract:

An "OS Not Found" error typically indicates that the computer is attempting to boot from a drive that doesn't contain a valid operating system or bootable partition. The presence of a USB drive might be confusing the boot order. Therefore, the first step a technician should take is to verify and adjust the boot sequence in the system’s firmware (BIOS or UEFI). It's possible that the USB drive is being prioritized over the internal hard drive, which may cause the system to miss the OS entirely.

A. Running data recovery tools is premature before confirming boot order.

B. Repartitioning the disk would destroy existing data—this should not be done until confirmed the OS is actually missing.

D. Switching between UEFI and BIOS (legacy mode) might help in rare cases, but it is not the first step in standard OS boot issue troubleshooting.