What happened to the barrier at the end of Digimon Tamers?
In terms of the Digimon regressing:


   It seems to have to do with their initial bio-emerging being undone. And, since Digimon in the real world are synthesized false proteins, it would make sense that, without the Digital World being properly connected, their physical form would not be able to remain, so they reverted their evolution and returned to the Digital World.


As for the portal appearing:


  Shortly before we see it, we see Takato running through the park, when a Digignome flies overhead. Since they are known to be able to grant wishes, in both the Digital and Real Worlds, it would make sense that it granted Takato's implied wish to see Guilmon again, opening the portal for him.