My team keeps on disappearing in Pokémon showdown. Why?
Teams are linked to the cookies on your browser. If you delete your search history/cookies, you risk losing your teams. If you use a different browser or computer, you also will not have access to your teams. It is recommended that you back up your teams using the Backup/Restore all teams button at the bottom of the «teambuilder». You can then store it on a site like PokePaste or Pastebin. The PS! desktop app is known to have issues with losing teams; we recommend that you use the website instead.
Source: /rfaq Lost teams in Help room. They are gone because you had deleted your cookies. Next time, remember to backup them to a website such as paste bin or a document.
Hope this helps!