[patterus2002]

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?