For future reference, there are a few things that can cause corruption. This potentially includes unexpected system shutdowns or crashes especially while a league is saving, bad sectors on the hard drive the game is saved to, memory glitches on your computer (especially possible if the league is left open for very long periods of time without the game being shut down or restarted) or occasionally interference from other programs.
So it's impossible to say which of those issues caused the problem, but some of what you might do to prevent that from coming up again would be to regularly defrag your hard drive as well as check it for errors, make sure to occasionally shut your computer off or at least restart it and also run as few other programs in the background at the same time as OOTP as possible.
For programs that need to be running at the same time, like an anti-virus or firewall, it's a good idea to add OOTP to their whitelist and exclude OOTP and its data folders from any disk cleaners or similar programs. This is a pretty important step, as it's probably where 95% of the issues with league corruption come from. Basically, any programs that can change or delete files on their own, without asking you about it, you'll want to have them steer clear of any OOTP files and folders.
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