My guess would be some sort of problem connecting to xkcd, or a temporary problem with xkcd's server (specifically dynamic.xkcd.com). There isn't (yet) any error handling there, so that would probably just give a blank text field.
For anyone encountering errors recently: Apparently the certificate at dynamic.xkcd.com was invalid, but c.xkcd.com seems to work so I switched to that. Things should be working now, hopefully.