How can I fix my gilera runner 125 ?

I have a gilera runner old shape 52 plate. It started everytime fine then I didn't drive it for a day and then I drove it for about 20 mins. Parked up and went into a house for two hours come out. Bike started but hardly had any power at all so I turned it off and it hasn't started since. It goes to start but jus hasn't got that last little bit to start the engine I bought new spark plug wich is sparking but still not working