you can also try a different set of frequency crystals if you have a set from another RC.
its rare, but the crystals can get damaged.

make sure the antenna on the controller is screwed in tightly.

if all that checks out and the re-charged onboard pack doesnt solve the problem then a new radio may be in order.

the flysky 2.4ghz radio system is only $20 on hobbypartz.com.