This issue was caused by the rollout of the change from bookmarks to saves, announced last month. TodayYesterday, the change began to be deployed and prior bookmark data began to be migrated into the new system, but today the process had to be halted due to site performance issues.
To quote from the most recent edit to the post:
Saves has been temporarily disabled network-wide due to some site performance problems. These problems didn't appear until partway through migrating existing bookmarks into saves. We've temporarily reverted back to Bookmarks.
- All saves, private notes, saved lists, and migrated bookmarks are unavailable. They are still saved in the database and will be available again when we turn Saves back on.
- All unmigrated bookmarks are currently available while Saves is disabled and Bookmarks is turned on. When we turn Saves on again, we'll resume the migration.
The migration was partially applied and will resume once we've fixed the performance issues and turned Saves back on.
- Some network sites were completely migrated, and your bookmarks were successfully turned into saves in the "For later" list. These migrated saves are unavailable until Saves is turned back on.
- Some network sites were not yet migrated at all, and your bookmarks are untouched. These bookmarks are available as they were before the Saves launch
- Stack Overflow was the only site partially migrated
- Around 360k bookmarks were turned into saves
- Around 14mil bookmarks are still bookmarks
We will post updates on this Meta post as well as on Stack Status.
As stated in the above update, the process of migrating old bookmarks to the new Saves system had to be halted, and the changes to the buttons on posts and user profile pages have been temporarily reverted. No data has been lost as a result of this action; it will all be available again once the process is complete.
As part of the process of migrating old bookmarks to the new Saves system, old bookmark votes were removed from the system. As such, if you're browsing a site where the migration process has taken place, your bookmarks list there will be empty as that page loads in questions with old bookmark votes which were removed (and replaced with saves). If you make any new bookmarks before the changes to post buttons and user profiles are redeployed, those will show up in your list, as you're casting old-style bookmark votes. At the moment, there's no word on whether these votes will be transferred to the new Saves system. If the migration process hasn't taken place on the site yet, your bookmarks page will be unaffected.
On Stack Overflow, the process was partway through before it was stopped, which is why some bookmarks are no longer present while others still are.
Again, no data has been permanently lost as a result of this process. It is simply not viewable due to the old bookmark votes being removed as part of the system and being replaced with saves, while the user profile page is the older version which only shows questions with legacy votes. Unfortunately, on affected sites, there is no way to see your prior bookmarks until the changes are fully complete.