It's been less than a week since Capcom rolled out Monster Hunter: World's highly anticipated expansion Iceborne, but for some it seems to have caused more trouble than it's worth. There are unofficial fixes though, and hopefully they work for you.
No doubt, Iceborne has turned into a cold shower for many players, no pun intended. However, it does seem to affect only those Monster Hunter: World players who haven't logged on in a while.
These players usually encounter the "Failed to read saved data for Monster Hunter: Iceborne. Create new save data?" notification, but it's imperative you don't save. Simply close the game without clicking yes to the "Close without saving" prompt.
Capcom seem to have updated their save system in the meantime, so you're also advised to backup your Monster Hunter: World save files, just in case. What this also means is that for some, the solution may be as simple as disabling Iceborne in properties, but others will require a few more steps.
Additionally, if you're encountering crashing during this process, you may want to remove the older Monster Hunter: World mods and trying again.
So, enable the console in Steam and type "download_depot 582010 582011 5080591846956782264", which will download MH:W's pre-Iceborne files. No need to worry if you don't see it in the download queue, just observe the active network usage.
You'll find your downloaded files in \steamapps\content\app_582010\depot_582011, after which you need to overwrite your existing ones. Start the game and when asked whether to save your game data - do so.
Once you're done with this step, go to Monster Hunter: World Steam properties panel's Local Files tab and click Verify Integrity. You should be upgraded to the latest Iceborne build, and your save will be fine.