Computer help for morons -- I'm getting a mysterious reboot error. Can someone help?

Hey all,

I’ve decided to ask my loverly fellow Jellies for help rather than try to decipher the results I got Googling my problem. I’m not a computer moron per se, but I don’t really know what I’m doing when it comes to fixing errors.

I was watching Netflix when my computer suddenly stopped doing what it was doing and now shows the following error:

Reboot and select proper Boot device
or Insert Boot Media in selected Boot device and press a key

Like I mentioned, I did Google the problem, but I’m having a really hard time understanding the advice I’ve found because I’m not super-techy and I just don’t get it.

Is anyone able to help me by giving me instructions in plain English (or by pointing me to a link that does)?

Much appreciated!