check that the crystals are in properly inthe transmitter & receiver, also try swapping the batteries for another set (they may be duds). If that doesn't work try swapping the crystals with a friends if you can, the crystals may be dodgy.

also has the receiver been dunked? it may have water in it & need to dry out.

before putting the boat back in the water do a range chack on land to make sure the problem is fixed before you have a runaway boat.