You might want to take it in and make sure it's not your alternator or the starter. Generally, engines won't just stop as long as you have oil in it, but take it back to your mechanic to have it checked. Ask if there is any type of warranty on the parts he used or the work he did. If you feel like you're getting screwed, get a second opinion and see if the diagnostics match up. If anything, order an OBD2 reader and use it to tell you what the problem is.

"The Engine" is kinda vague. Like, what's wrong with it? There's a lot of really simple things that can cause a vehicle to not start, that are pretty easy to fix.

Honestly, I think it would be a good idea for you to learn a bit more about how cars work, it's extremely useful for anyone to know at least basic things about their vehicles.