engine wont start

I feel pretty sick right now. My "check engine" light came on about 2 weeks ago but the truck was running well and I figured it was nothing serious. I thought I would get it checked out when I changed the oil,which was coming up soon. I went to start it the other morning and the engine cut out as soon as it started. When I popped the hood I discovered that the oil cap was not on. The oil level is about half a quart under max. The only thing I can imagine is that I did not put the cap back on when I changed the oil the last time. Have I ruined the engine??? Is there anything I can do at home to diagnose the exact problem before taking it to a mechanic?

I wouldn't immediately say that you have ruined the engine, but when it comes to diagnosing at home, I wouldn't worry too much with it, and take it to a mechanic and have everything checked out. It's gonna cost a little $, but if you want to keep your truck running, It is the best thing to do.

What can I do to get it started?

I am not sure honesty. I wouldn't think the oil dropping a half a quart would cause problems. It could be any number of things. I would get a buddy with a code reader, or get one loaned from your neighborhood AutoZone or part store and go that route. If the C.E light came on, then there is definetely a problem more serious then a slightly low oil level. When it comes to starting it, I wouldn't know where to begin, at least not without knowing what is ailing your truck. Get a mechanic to come over and work on it, otherwise, Tow Truck to the shop, my friend.

the oil cap most likely has nothing to do with this. go to autozone and get it scanned before you pay for anything. then take it from there after you find out what is exactly causing this.

most likely there is a dirty sensor or a loose ground. clean youre MAF asap cuz it sounds like a 50% chance thats whats wrong.