Windows desktop computer

@voilet12 

There can be a few reasons, from a dead hard drive to a motherboard battery needing to be replaced. Others too.

Google "an operating system wasn't found dell" and you'll see many web pages with this problem.

Here is a Dell KB article on this problem for instance, https://www.dell.com/support/kbdoc/en-hn/000130881/dell-pc-does-not-turn-on-or-boot-into-windows for some steps.

Depending on where the problem lies, steps will be different. Even for the possible OS you have it it is not a h/w issue.

Always include exact PC model and version of Windows in your posts...