I got this leak all of a sudden, and it was coming from what looks like a missing bolt on top of the intake manifold. This is what it looked like right after I turned the engine off, and the picture below is the hole after the engine cooled down. I have little knowledge of mechanics, I took it to the shop and they said they'd needed to replace the entire manifold. What should I do? Should I but a replacement bold and put it in myself? No idea what happened here, any advice appreciated.

This is what the hole looks like

 

Those bolts don't just decide to go south.
That "opening" does not appear to be a bolt hole. At least to me.
What you might consider, get hold of a cooling system pressure tester and apply some pressure and see exactly where that leak is.
DO NOT APPLY OVER 15 PSI WITH THE TESTER, OR YOU COULD CREATE OTHER LEAKS IF THERE ARE ANY WEAK SPOTS.
Once you are sure , you can decide on what measures to take.

 

Thanks, that's what I needed to know. Much appreciated.