If you used the testnet you can search for the contract's address (or the address used to broadcast it) using [etherscan's testnet site][1]. If it doesn't show up as a contract creation transaction then the contract was never deployed.

If it was deployed you need **both** the address of the contract and the ABI (for example [this][2] is the ABI for the DAO contract). 

Using these you can open geth from anywhere and have a local instance of the contract with:

    var abi = JSON.parse(contracts_abi_string);
    var myContract = web3.eth.contract(abi).at(contractAddress); 

 

You can then interact with `myContract`.


  [1]: https://testnet.etherscan.io/
  [2]: https://gist.github.com/fivedogit/4f3e9a035262b99b2e7356a4fdf6dd34