I and a couple others set up alerts on a site (two different site collections were involved), but they disappeared. (It's a little confusing to talk about all three of our experiences at once, so I'm going to share mine - the other two experienced basically the same.)
I had an alert set up on two lists in my site and was receiving weekly notifications. One week, they stopped. I checked the "My alerts" page, and they had disappeared. I definitely didn't delete them, and I'm the SME (subject matter expert) on our team, so no one would go in and delete them without asking me first.
My question: do alerts expire after a certain amount of time? If not, I'm guessing it was a glitch that our SP admins caused, but they didn't realize it.