Requested by @ameliaville. This varies, of course, from employer to employer, but I can tell you how it works at my tech shop.
  1. Day 1: You don't show up.
    On Day 1, I'll generally give you the benefit of the doubt. But don't be a jerk about it; let us know if you're going to be out!
  2. Day 2: Concerned e-mail and voicemail.
    "Hey, so-and-so, we haven't seen you in a couple of days. Are you okay?" If you reply, you'll get a sternly-worded reminder that we expect you to let us know you're going to be out and that you need to get back to work.
  3. Day 3: The big guns.
    "Hey, so-and-so, we haven't heard from you. If you don't contact me by noon tomorrow, I'll call the police to come check on you." This threat is very, very effective. If you reply, you'll get put on a performance improvement plan for about six weeks.
  4. Day 4: Hasta la vista.
    If I still haven't heard from you after the deadline I set, HR terminates you for abandonment of job, and I get to call the police to do a well-person check. I've been through this process more than once but have never gotten to this step. Threatening to call the police always gets a response.