I looks like they didn't clip together properly. They may have come apart because the surface underneath isn't flat. If this is the case, dissassemble the floor and pour some floor leveler. If it's cheap flooring the locking system may be inadequate and just didn't hold together.
You can try to tap the boards together with a hammer and the pry tool that you used to install the boards. However, that only really works if the boards are properly connected but not fully seated.
It looks like the two in the pic are not connected at all and they're just sitting next to each other. In that case you'd have to disassemble the floor and make sure to attach them correctly. It's not terribly time consuming if you don't have to cut any new pieces.
The cheap fix would be to get the color-matching stretchable filler, but those are more used for holes; I think it might introduce more problems when the floor shifts.
Wood flooring needs to acclimatize to the environment for a few days before being installed (shrinks/swells with the moisture in the air).
Vinyl flooring are designed to latch together. Every board has to be completely seated and latched together. The only way to fix it now is to pick it all up and try again.
Have someone stand on the gap of the two boards (they should be ensuring the two boards are level with each other.)
Take a pry-bar (crowbar) and push the flooring from the edge (may have to remove quarter-round that is covering it).
Basically you are pushing the entire floor to get the boards to snap together. The person standing on the gap helps to ensure the boards are level with each other and prevents the other side of the flooring from moving. It can take some trial and error to figure out. You want enough weight on the opposite side so that the planks will snap together.
I haven't worked with vinyl planks so not too familiar with them. With the wood ones you had to be careful not to break the tongue sections.
There are methods to tap the boards around, Google should give you some viable options on how to get the "grip". I think using duct tape was one of the easier options.