I had silicone implants 30 years ago. My right breast has become hard. What is it and is it correctable? Also is it dangerous?

You have old silicone implants that are hardening - know as capsular contracture.  This is the scar capsule around the implant that can harden over time. These old implants are most likely ruptured - for some time- but contained within the thick scar capsule - and not outside it, which is extra capsular rupture and this can be palpable and very visible on mammograms. This can be corrected with excellent results yielding soft breasts and implants with a natural shape. The capsules and implants  are removed and replaced with new silicone breast implants preferably in a new pocket or location, submuscularly is my preference.  You should be able to obtain breasts like you once had. With new shaped high-strength silicone implants results are natural and have given my patients a new found satisfaction with their breasts. Implants that are form stable and anatomically shaped also greatly diminishes the risks and concern of rupture to almost nothing in my opinion. 

30 years is a long time to have had your implants, which is great if you have not be having any problems with them until now.  Because they are 30 years old and your right breast is hard, your implant is most likely ruptured which may have caused a capsular contracture.  You can have an MRI but at this point, it would be best to remove and replace your implants with new silicone gel and also have a capsulectomy to remove that scar tissue.  You want to remove them as soon as you can because it can get worse and more uncomfortable. Make sure you see a board certified plastic surgeon as well.  ac