White Forest (along with its counterpart, Black City) is a bit complicated to explain, but allow me to try.
When you turn on your game for the very first time, White Forest is a lush, vibrant area populated by 10 NPCs who bring with them various Pokemon to catch and items to find. These 10 NPCs are randomly determined from a list of 30 possible NPCs. However, as you know, you cannot visit White Forest when you first turn on the game. Here inlies the problem:
Each of the NPCs who "live" in your White Forest have a Happiness level which decreases every day that you don't visit White Forest and talk to them. So, from the moment you start the game until the moment you beat the main storyline and can get to White Forest, your NPCs are in danger of deserting your White Forest from lack of interaction. To make matters worse, the less NPCs you have, the less lush and vibrant White Forest itself will appear. So, without you even knowing it, taking your time playing through the main plot of the game (or "time travelling" to force-start various seasonal events) has had a terrible effect on your White Forest. In your case, to the point that ALL 10 of your NPCs have departed and left you with no grass or water to find Pokemon in.
Alas, there is no way to get them back. However, all is not entirely lost. If you can find (locally) someone with a copy of Black Version who has already accessed their Black City, you can visit their Black City via the Entralink and bring their NPCs into your White Forest. This will not make them leave Black City, but will simply copy them from there into your White Forest. The more NPCs you "recruit" this way, the larger and more developed your White Forest will become. Keep in mind that you are capped at 10 NPCs, so there is no way to get all 30 of the possible NPCs at once.
For more information on the NPCs, their Happiness Levels, and the Pokemon and items they bring with them, refer to this link from Serebii: