An operating system was not found.Try disconnecting any drives that don't contain any operating system, Press Ctrl+alt+Del to restart
It's because of installation when u kept dynamic or due to because of partition problems.
Alternate solutions u can try is Bios Settings update
Reference link: https://www.youtube.com/watch?v=zO7linfqcuU
Best solution: DO Fresh install the system by taking backup of your data through Live.