I'm running client version 0.3.14 and there appears to be a disappearing balance bug.
If I "lock" my account, unlock another one (e.g. a new one), and then go back to my original account, my balance drops to 0 even though my transactions are still listed. Even if I close and restart my web browser, the balance still shows as zero.
However, if I stop the Java process and restart it, everything returns to normal.
The good news is that it appears to be a GUI bug, and not an issue with the blockchain or account.