to start with, what email client are you using? was it a web based client like Gmail, or was it something like the MacOS mail client or a eM client that you installed on your computer? If it was something like gMail where you went to a website, it was more than likely that you cleared your cache to fix some other problem and over time it will come back but in the meanwhile you are probably connected to your contacts if you store them in the gMail contact/address book (contacts.gmail.com). Then it's just a matter of hitting on the "To:" link and selecting them from the list.