You would get a disease called scurvy, which was a common problem on ships prior to the discovery of vitamin C, because it was difficult to get on ships (generally, you needed fresh fruits and vegetables, and those tend to go bad on ships). This is the origin of the pirate expression "scurvy scalawag". Scurvy causes a number of problems, including teeth to fall out of gums, and other bad things, eventually, death. I don't know how the disease progresses.

If you don't have enough vitamin C in your body for long periods you can develop a condition called Scurvy. This was found often in sailors when they would be at sea for many months without access to fruit or vegetables. Scurvy causes the collagen in your skin (the stuff that makes your skin stretchy) to break down, damaging your skin and gums.

Some of your questions about Vitamin C sound like good Science Fairprojects. You can start at the highest temperature, and see how muchVitamin C is lost. Then you can test lower temperatures. The questions about sugar and freezing are also questions you can answer in your science fair project.

If you don't have enough Vitamin C in you, you get a disease calledScurvy. You can find the details on the internet, but I think your gums start to bleed a lot.

Different vitamins will break down different amounts when you heat them or boil them. That is because they have different chemical structures that are more or less damaged by heating.

Do you have a good way to measure Vitamin C for your science project?