Is the Testnet stable?
Posting my comment as an answer

Make sure your client is up to date... it's possible your wallet in the old testnet got coins that is separate than the more modern testnet3.

Each version of bitcoind has the possibility of being associated with a different test network.  It would be interesting to see if there was a matrix listing releases against test networks (and why the new network was created).

I assume that it's possible to launch older versions of bitcoind and see each testnet independently of each other, but that independence means that there is no way to transfer coins from one network to the other.  (It is test after all, and hopefully one didn't pay for those coins)