Probably your configuration file (prime.txt) also was lost, so why user id was empty and server address set to default GIMPS. You could restore this from client archive (just overwrite existing one and set your username).
You may TRY to continue already completed work if you have checkpoint files, named something like Z12345_678912345 (the first letter may be different, I don't remember which one exactly). Naming of them is very simple - first number is "k" and second is "n", so this file would mean workunit "12345*2^678912345+1".
Find out which checkpoints you have. Then, go to http://seventeenorbust.com/account/testsPending.mhtml and check that these workunits are still assigned to you.
To recover, get a copy of worktodo.txt from another computer. If you don't have one nearby, just let client to connect to SoB server and let it crunch for a few hours until it completes phase 1. Stop client before making any changes.
Open worktodo.txt in simple text editor like notepad. You shall see some "PRP=...." lines. Their structure is very simple, it's easy to find which number corresponds to "12345" and "678912345" parts from example above. Duplicate, delete, move these lines as you wish, substituting "k" and "n" from names of checkpoint files you have. You may have any number of PRP=... lines, client will process them sequentially. When you start the client, it shall resume partial work from checkpoint.