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Without more information its hard to say, this can happen for example when you clear your browsing history or local storage. You could try looking in your local storage if there's still something there but it sounds like not.
If you still have your browsing history try looking at all the links with pokepaste in them, you might be able to find your teams that way.
The best way to prevent your teams from being lost is just to do a backup regularly. Click Backup/Restore all teams at the bottom of your teambuilder, copy the text and paste somewhere safe, or maybe put it all in a pokepaste and save the link to that pokepaste somewhere safe.