Make sure your account is valid, just try to login to the website with your account/password... If you can login then you only need to download the patch, or new client.
If you cannot login to website !, (this will happen if your accountname is the same on funzmu/silvermu or ancientmu)
use this to fix: http://www.ancientmu.com/acc_rename.php ( just login with your accountname + password... and make a new accountname (write it down don't forget )