Today a bunch of people on my contact list received a spam email that appeared to be from me. I never sent this email out and I don't see anything on my "sent box". What should I do? Change my password? I don't really know what happened. Any help would be appreciated.
Define "appeared to be from me".
Most of the fields in the email headers, including the "from" and "return to" headers, are freeform text fields. Spammers can put anything in them they want. Sometimes they choose to put the email address of other spam victims (you).
Have your friends send you copies of the emails that include all the headers. From there you will be able to see if the emails came from your box, or at least your ISP's email server.
A good place to learn about this stuff is http://www.SpamCop.net