White Forest

Just ask away.

don't be shy to ask away man, there will (almost) always be someone who can help you out

have you messed with the clock? that could be a reason,
and (i've read) if you take too long to get there

ifyou connect to a pokemon black through entralink, there should be grass in there

quote Disillusioned

If you don't talk to the NPC's there everyday, they and the grass will start to disappear. Every NPC is connected to Pokemon so if you have that NPC, you have their Pokemon in the grass.
But if you arrived there and found no grass, you took too long. ):

But if you have a friend with Black, using Entralink with them will cause your forest to grow again.

_{So that's why D:}

ok thank you guys i beat it and got there and there wasn't anything there but it took my friend 16 hours to get to white forest thats pry why he has grass and water in white forest lol