Your peugeot 306 is turning over but wont start up itook 1 of the relay fuses ot and when i put 1 back in it wont go?

you need to check the relay is back in correctly. if it still dosent start you need to check you are getting power to the fuel solenoide on the fuel pump (For diesel engine). for a petrol you need to check for spark and that fuel is going in to the cylinders.