Error code 999. i can log on, but then an not access anything. how do i fix this problem?

Clear the cache of temp files and cookies, restart the computer. If that doesn't work, sign out, shut down the computer for a few minutes. When you turn it back on, turn off the modem for a couple of seconds to a couple of minutes, when you turn it back on, sign in and it should be cleared. If not, you will have to wait for a time. Sometimes a couple of hours or more. This is just a code that Yahoo puts when the problem is unidentified. It's generic