Well This can be a Number of reasons.
1. Once you login, and it goes to the "Click here to play RuneScape" button and other Miscellanious stuff, and does not load properly, that is just a basic simple lagg.
2. You Login, then it goes to the Miscellanious Screen and You login, and the Screen is all Darkened, That Means there is somthing Client-Side effectiant Error, that can be improved with a new client.
3. You login, but the Screen goes Completely White, this is Server-Side and you must try another Account, Meaning your account is Unable for use.