Seems like you live in the U.K. so I'm not sure how you would go about getting the code determined, but here in the US most auto parts stores will read the code and you can take the apropriate action from there. You see, the OBD2 ( on board diagnostics) system will diagnose itself and the information can be read from your cars computer with a code reader. You can buy one or borrow one.
If you blazer is a 96 or newer you have OBD2.
Check your gas cap. If the light came on shortly after a fill-up, it could be that the cap is loose. Although this is frequently the cause on newer vehicles, it's worth a look on your 2000 model.
Other than that, as RobertBrown says, you need an ODB diagnostic to tell you exactly what it is. With an older vehicle like yours, if it's not missing badly, could be emissions sensors. If it's missing, could be plugs, wires, coil packs, injectors. The "Service Engine" light is not something I lose sleep over, like an Oil Light coming on. LOL.