#1 Check your spam folder. Check your deleted emails folder. Some deleted emails aren't actually gone til you flush the toilet, so to speak.
#2 If you did delete your email with the key there is no way to get it back from SE. You can try to angle for one of the keys that gaming sites are handing out, but they seem to be going within a matter of minutes (I tried and failed for a PS3 key on one site, and ZAM was cleaned out in like thirty minutes yesterday.)
#3 Try doing a search for "beta" in your inbox. I missed my alpha invite at first but found it later by searching my inbox.