Why isn't my iMessage working!

Okay so I went away for a few weeks and now that I'm home I want to use iMessage on my phone again (iPhone 3) but it was off so i just thought that it turned off automatically when I went overseas but when I turned it on again it wouldn't work; it just sat there with "waiting for activation… " on the screen and not actually working. So I'm just wondering WHY this is and how I can fix it?

Try texting me…