Firstly don't panic!
30% bone loss isn't great. However 30% is a level where if you get your gum disease under control you can keep all your teeth and basically never have any problems.
Contrary to what Mummy says we get concerned about any bone loss. It depends how old you are really, if you are 20 then it's very concerning, if you are 80 then not so much.
Also just because you have bone loss doesn't mean you have any active disease, it could be historic. I would want to know more about the condition of your gums than bone levels, you can't really go off bone levels alone as it doesn't tell you if there's actually anything going on. Only that something has in the past
What's your oral hygiene like? Did your dentist talk you through a proper oral care regime? Are you flossing or using ID brushes daily?
Bone loss is caused by gum disease. It is caused by plaque, smoking or diabetes normally, or a combination. Rarely other things
If you don't smoke then it is most likely down to plaque build up. In which case the dentist should be able to help you identify areas you are missing and ways you can improve. As well as this having regular scales with the dentist or regular hygienist visits