After a firmware upgrade, it's usually wise to do a factory reset of the router, in case the upgrade caused any problems. The reset can be done by holding down the rest button for 30 seconds. Be aware you may be asked to re-enter your Verizon credentials in order to re-connect to the Internet. Any other customizations to the router would need to be re-done too, if you don't have a backup copy of the configuration file. If you're unable to log in after setting the new password then this is the only way to reset said password quickly.
Beyond that, the simple solution is to try rebooting the modem. If the web interface just crashed after a password change, then there should be no harm in rebooting.