It's a known problem, and there are some causes. It's basically to stop pirated copies from selling.
This will happen if your game is:
3. on a mass storage cartridge like R4
If it's ROM'd, Google for a exp. patch for White 2.
If it's pirated, get a real copy.
If it's on an R4 cartridge, bring it back to your seller & ask them for help.
Restarting the game can solve the problem, but it's not guaranteed.
If it's a genuine copy of W2, take it back to your retailer & ask for an exchange.