There's a "disclaimer", at least when you first open the private image gallery, saying that the gallery is stored in your device only, on your browser cache. The problem: browser cache is often erased by the browser itself to clear up space.

For that reason, you must always save your images yourself, either in your device, or on cloud. Given the nature of your image, and many other reasons, I recommend always using local storage, since cloud services tend to be too annoying with what you save there (Google for example explicitly forbids saving NSFW on Google Drive, and anyone who does so is only surviving through luck and they may lose their files at any time).

In the end, this looks like to be a case of it, although...

Upon searching "internal error opening backing store for indexeddb.open" on Brave, their AI gave the following forms of prevention:

• Avoid force-closing apps during database operations.
• Ensure sufficient disk space and proper file system permissions.
• Use app.requestSingleInstanceLock() in Electron apps to prevent multiple instances.

And the fixes often included restarts, reinstalls, clearing cache, and many other things that also including "erasing things".

Upon looking at your screenshot, I notice that there are 99 tabs open, and there's a chance there's more and this is only 99 because of the limit in digits. And based on the kind of people who does it, there's also a considerable chance that you also have many unused apps and even "open but not being used" "windows" on your device, which makes the situation worse.

Therefore, I have the theory that the issue was likely caused thanks to the sheer lack of space on your device/browser, which caused perchance to glitch and erase all your things (in case it didn't come back after countless restarts).

Therefore, besides "start saving your images on your device properly", I believe that you should also close this absurd amount of tabs, since they may cause many problems. In case you want to save them, bookmarks is a way better way to save things you want to take a look later.

oh no! you lost your masterpiece, best quality, ultra detailed hentai illustration seductive bride! i can't imagine how upsetting that must be