IPv6 is working, but IPv4 isn't

Not sure if this is a permanent solution, but I managed to get things working again by releasing and renewing my IP address:

sudo dhclient -r wlp2s0
sudo dhclient wlp2s0

Now I see an inet addr line when I run ifconfig (which was never there before) and cat /etc/resolv.conf has added an IPv4 nameserver and a "search" line for my ISP (search hsd1.va.comcast.net). Problem solved!

Sometimes this can be caused by an incorrect system time. If your system time is too far off, you will not be leased an IPv4 by DHCP -- only an IPv6.

You can check the system time settings by running timedatectl in a terminal.

If you need to reconfigure your system time and/or time zone, run sudo dpkg-reconfigure tzdata in a terminal.

Assigning a static IP/Gateway/DNS could also work regardless, but certain websites will complain.