what is wrong with my megane 1.6i 2003,check injection on dash is on?

i have a renault megane 1.6i 2003,on the dash it says check injection and the car has no power.it wont go over 10 mph!! i have changed the coil pack but thats not helped.kwik fit and nationwide say they wont touch it,go to a back street garage.im quite worried this will cost me a fortune.any suggestions anyone????