I have never experienced it personally, a couple of the senior members have though.
From what I understand you just have to help them along a bit. They still have a mouth. Cup feeding helps them catch food. It's just like a person without a limb. It's challenging to adjust, with some love and support it will be overcome.
In the wild they would probably not make it, but there have been several members on here who have had chameleons live a good life without a tongue. It is just the pain of having to hand feed them to make sure they are eating.