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. If it doesn't show up as a contract creation transaction then the contract was never deployed.

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. 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 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.