A villager was planting a tree. I just checked another tribe and went back... the seed was gone. How can I get it back?
P.S. In an earlier post I said I lost all the VV3 tribes, now I can never get them back so I started new game

If you quit the game or switch tribes while a villager is carrying something like a seed or collectible, it will be lost. Carried items are not stored until the villager completes what they are doing, so make sure that you let villagers finish carrying something before you leave the game.

What to do now?

You'll either have to restart the game or wait until you solve some puzzles to increase the food supply. If I were in your situation, I would restart the game.