Mazda 3 problems.. Check engine light on, nothing appears wrong?

I drove to work today, it was fine. I got in my car at lunch, and the check engine light was on, and nothing appeared wrong. I immediately get ulcer- bad stressed. I am broker than broke and can't afford any extra expenses. I checked the oil, which was almost completely gone (I'm not kidding about being broke, I haven't been able to afford an oil change in 7800 miles). So I go and charge my oil change, and the light is still on. I would love to get a hold of and OBD II reader, but you can't rent/borrow them in California anymore.