First, hugs. It's very painful to have a book go unsold after finally getting an agent -- heartbreaking, even. It has happened to many, including me.
I agree with the most of the above -- get the information on the book's submission history, but likely you will not be able to get another agent to submit it unless they take you on as a client with another book that hasn't been shopped.
One thing I don't agree with is the characterization of your book as damaged goods. A book is not damaged by being read, but it does have fewer sales opportunities left. Although your book didn't sell, that may not be the agent's fault, nor a flaw in your book. It's tough to break in with debuts. If your agent is forthright with you and a good advocate for you and your work, when you have another book or proposal, send it to him or her. Hopefully the agent will love it as much and will rep that one too.
If you don't feel you were treated fairly or you don't trust your agent, query agents with a new book. Or, since your contract has expired, you could sub your first book to publishers yourself, if you know who has already seen it.
Good luck.