2005 Ford Focus My Check engine light is on

Engine Performance problem
2005 Ford Focus 6 cyl Automatic

I noticed thursday night that my check engine light was on I checked my owner manual to see what could be the problem. After referring to the manual I did a few drive cycles as it said it could be fuel related and also checked the gas cap. Non of the above seemed to solve the problem. I month or so back my check gas cap light was on but after a few days it went off. Im not sure what caused it or what's causing my check engine light to come on. I recently had to replace the alternator because it went out after changing the battery thinking it was that. I think I know my car well enough to know when something is wrong with it thats why i'm baffled to why the check engine light is on there seems to be nothing wrong with my car. Any ideas?