Depending of how bad your "crash" was....and if you have Windows, you can try to access "reset" . Follow the directions. IF the computer is completely unresponsive, you'll probably have to enlist the help of the Geek Squad or some such tech help.
Typically, a missing operating system error can be caused by a hardware failure, such as a hard drive, or by a software error that your system is experiencing. Errors when loading Windows can be very different, I use 4 Useful Methods to Fix Missing Operating System , which describes the most common ways to restore the operating system, everything is scheduled step by step, I think you can handle it yourself.