Some Baldur's Gate 3 players are reporting issues with the game's save system. Namely, that they can't save or load at all, which is a bit of an issue in 2023's Game Most Likely to Spark Discourse About Save Scumming. The good news is that, if you're running into this, Larian is directing you to a help page on its website that should hopefully solve the issue. The bad news is that you aren't getting any of the progress you lost to a busted save back.
The issue seems to be tied to BG3's cloud sync service for its many, many save games. Players encountering the bug report the game becoming trapped in an endless sync loop, preventing them from saving or loading while the game works fruitlessly to offload data to the cloud.
Larian's support page for people unable to save recommends deactivating "all non-essential programs" before you launch the game, "especially any anti-virus and any backup programs" that might be interfering with the game's ability to muck around with its own data. It specifically calls out Windows Defender, Bitdefender, Avast, and AVG as programs that have historically stuck in BG3's craw, and recommends that users add the game's executables (and the Larian Studios AppData folder, which you can find at \Users\<UserName>\AppData\Local\Larian Studios) to those programs' trusted exception lists.
If none of Larian's recommendations are helping you out, some players on the Steam forums report success in just deactivating cloud sync entirely. Those players recommend turning off the game's "Cross Save" setting under its Gameplay Settings before wholesale deleting the Baldur's Gate 3 folder in AppData/Local/Larian Studios. Taking cloud saves out of the equation should, hopefully, get the game back on track. Although it might come back to bite you if you try to play on console at some point in the future.
Hopefully, it won't be a problem for long. Larian told IGN that it's aware of the issue and Larian CEO Swen Vincke tweeted earlier today that the game's first hotfix is already well on its way. With any luck, this won't be an issue for players affected by it for long.