To fix An operating system wasn’t found error on Windows computers, first check if the Disk Type is GPT or MBR, and then follow these steps.

I have solved the "No operating system found" by replacing the boot.ini file with a known good boot.ini. Not sure if that would work with your problem.

If you see Error 1962, No operating system found, Press any key to repeat boot sequence when you start your Lenovo computer, see this post.

Whether you're on a Windows PC or a MacBook, it's easy to keep tabs on your operating system.