I've been asked this quite a bit so I thought I would leave a little guide here:
Sprytile displays all the tilesets you currently have added, even those the object you are working on doesn't have a material for. If that is the case, the tile selection UI will not show up (In 2.7 it would just show a blank image). To bring it back you can either:
1) Add the tileset again to the selected object with "Add Tileset" as usual (This will create a duplicate)
2) To reuse the existing tileset, go to the material panel with your object selected, add a new material and set it to the tileset you are trying to use:
(Here for example I'm adding terrain_tileset to the object)
When you reselect the tileset, the tile selection UI will show up.