Monster Hunter World: Iceborne finally arrived on PC yesterday, but it's not going especially well for everyone. A number of players on Steam and Reddit are complaining that the update has deleted their saves, which in some cases has cost them hundreds of hours of gameplay.
After installing the Iceborne expansion, players impacted by the issue are told that a compatible save file cannot be found when the game starts. At that point, they're prompted to create a new save, which begins a brand new game. Making matters worse, according to VG247, exiting the game at that point automatically uploads the same to Steam (assuming you have Steam Cloud saves enabled), so you can kiss that one goodbye, too.
One theory is that the presence of certain mods could be causing compatibility issues, although at least a few players have said that they lost their saves despite having no mods installed. Another possibility is that long-absent players may have missed an update that changed save formats, and Iceborne is unable to read the older saves. A Capcom rep said the team is aware of the complaints and looking into the issue, but hasn't come up with anything definitive yet.
If you haven't yet run the Iceborne expansion but plan to do so imminently, you'd probably be well advised to back up your saves manually before you do anything, which you should be able to find in Steam\User data\(your steam user ID number)\582010. If your saves have already been wiped, you might be able to restore them with the MHWSaveTransfer utility and this "Iceborne Ready" save, both available from NexusMods (via Steam user nomotg).
Alternatively, instead of diving face-first into a potentially risky DIY fix, you could just wait a bit while Capcom sorts it out. You do you, but that's the route that I'd recommend. We'll let you know when more information becomes available.
In the meantime, if you've avoided any issues and are happily hunting monsters in this icy new world, we've got some helpful resources you should probably check out, including list of fearsome new beasts, ten things you should do before you get going, and a Guiding Lands endgame guide.
Update: There's no word of a resolution yet, but a Capcom rep said that updates will be posted when they're available on the Steam announcements page.