There are a couple of possibilities. One is that the battery got very low, so the phone shut off to preserve memory. Normally it will come back on after 15 minutes of charging, however. The more likely cause is the operating system crashed (sort of like a Windows Blue Screen of Death). This can be caused by memory corruption, too many apps in memory, or bad data received. It's fairly common, and rebooting will fix it. One way to reduce the chance of it happening is to turn off the phone for a minute or so every few days, or perhaps once a week. To do this hold the SLEEP button until a slider to power off the phone appears. Wait about a minute after sliding it, then press SLEEP again to turn it back on.
Holding the 2 buttons reboots the iPhone. The trick is to think of it not as a phone, but as a powerful pocket-sized computer that can also make phone calls.
And Happy Thanksgiving to you, also.