Seems like they have generated wallets offline, with hash-parameter links. (wrong).
Do you still have a bookmark with an Instawallet link?
The URL would be something along the lines of
https://instawallet.org/#ABigBunchOfJumbledCharacters.
They were an horribly insecure online wallet, and got hacked (or that's just an excuse).
Anyway, see this thread:
https://bitcointalk.org/index.php?topic=167215, which was created by the developer of Instawallet.
Don't post your wallet link there, though.