I created a new deck for studying and when I uploaded it I thought it would add it to the decks I had on other computers… Not sure why it decided to delete everything online? Can I restore the files I had before?
Did a message come up asking you to choose between uploading and downloading? If yes, then you’ve done a full sync. This is required when you make certain changes that force you to choose either to replace your collection on AnkiWeb or your local one.
If you don’t have the other decks on the computer you made the new deck on, then I think that’s the problem: You should make sure to keep your devices in sync by avoiding making changes on one device, then making other changes on the other device without a sync between them.
If you still have the old decks on another device that you have not synced to yet, you can export the decks, then sync normally to get the new deck to the other device, reimport the old decks, and sync again to make the decks available to other devices again.