Greeter contract, greeter.greet() gives error greeter is not defined after re opening geth

trial2try

I am new to ethereum,
I executed the greeter code [https://www.ethereum.org/greeter] in my private network and when I called greeter.greet() after mining. And it returned Hello world.
Later I closed my geth terminal and then re opened. This time greeter.greet() threw me an error greeter is not defined.

How do i call this greeter.greet() of my contract without redeploying?
How to permanently store this greeter variable in my geth ?