Flickr was acquired from Yahoo by Smugmug last year, and the photosharing service has imposed much stricter limits on storage for its free tier of accounts. Free Flickr account users can no longer store an essentially unlimited number of photos and photo albums in the previous 1-terabyte account limit.
All photos beyond the 1,000 images allowed for free users WILL BE DELETED, not just no longer visible, starting this week.
If you have photos stored only on Flickr, it would be a good idea to download them and archive them locally. If you have web pages or blog posts that embed photos directly from Flickr, those embedded photos might vanish.
If you're using Flickr albums to display content on the web, be aware of the new limitations. I've always encouraged people to upload SEVERAL select images from a given event, rather than dozens or hundreds, and that helps keep the account to a reasonable size.
Note that if your photos were uploaded with a Creative Commons license, or set to a CC license prior to November 1, Flickr is letting them stay.