@cmb:
Ok that rules out the IP conflict theory.
What type of WAN do you have, DHCP, PPPoE, ? My next guess is something to do with the WAN not connecting (because the modem hasn't synced up/connected yet) is hanging up PHP, which prevents the web interface from working.
I have Cable DHCP
I'm thinking maybe your explanation is the only logical one. But I shut the power off and then back on. and left everything for 30 minutes, and I still could not log into the PfSense box, could ping it but not log in and no access to internet.