why is the emissions light on my astra 2001 ?

why has my emissions light come on my 2001 vauxhall astra 1.6 ? i have had a some one out to look at it and had it connected to computer and cylinder 3 misfire detected ! he took the light off but after driving around 5 miles car was left overnite not used then started from cold drove 1 mile light came back on car was struggling a little ! but drives ok now , the light is still on