It happened again today.
Not sure if it is related, but my battery was pretty low this time (10%).
A reset right away didn't work (even when I started charging), but after I let it charge up while off, the network came back when I turned it back on.
I'm starting to think this is a battery issue (the phone is old).
Is there an easy way to test (through the bb or with a voltmeter?) to see if it the battery gives too low a current when it is near empty? Or something along those lines?