Is there some sort of auto shutdown feature
No. Most likely the pi is still running, but has gone offline. As Steve points out, this may be because of insufficient power for the wifi adapter. Problems like this will not necessarily show up in an either/or manner.
I plugged the hdmi cable in and there was no display on my monitor.
Does your monitor normally detect "No input"? E.g., if you plug in an HDMI cable with nothing on the other end, will it say that? If so, did it say that when you plugged the pi in, or was it just a blank screen?
If I plug an HDMI monitor into a pi that booted headless with no GUI running and
hdmi_force_hotplug=1 set in
/boot/config.txt, I get a blank screen, but not "No input". Plugging in a keyboard and trying to wake the display up/switch VTs does nothing. However, when I unplug the HDMI cable from the pi, leaving the other end in the monitor, I get "No input".
Raspbian wheezy by default dumps all system messages to
/var/log/syslog. These are timestamped, so if the time on the pi was set correctly via
ntp and you remember approximately when this was, you could look through that for clues about the state of
wlan0. Boot is indicated with a message from the logger,
rsyslog, indicating "start". Then there's a dump of the kernel message buffer (like
dmesg) from boot (which preceded starting the logger). All these messages will likely be stamped
Note that if the system is idle, there might not be any messages, so unless the system is shut down properly, there will be no indication of when it stopped.