Check engine light: one of cylinders isn't fireing

I just had a "check engine" light turn on in my '03 WRX wagon. It has 27,000 miles on it. Took it in and the dealer wants to keep it a couple of days. Said one cylindar wasn't firing properly. Does anyone have any ideas what could cause this type of problem? If it is to be worked on for a couple of days, aren't we looking at some major engine work (versus a computer chip replacement)?

Any thoughts would be appreciated.

Thanks,

Kim