Peugeot Speedfight 2 won't start?

My daughter has just brought an 03 Peugeot Speedfight 2 but it won't start. It was O K for the first couple of days and then wouldn't start. She put in more petrol - although it wasn't empty - and it was alright for another couple of days but since then nothing despite adding more petrol and oil. It seems pretty dead so could be the battery or plugs - any ideas ?