Could be a lot of things: If you went inactive then it's understandable that you might have to warm up your account again with regular posting.
If your boards stopped attracting new followers, it's kind of inevitable that at least some of the old ones would have fallen off.
Engagement numbers would be really helpful to try and figure out what's going on.
engage with your always like pinning what they seek.I have Pinterest account with over 16k followers - they was very active a few month ago but today not - why??