You probably have a Web/SSH restriction applied in the Gaia configuration of the SMS. If you can get to the console, from clish do a show allowed-client all to see what IP address you need to be coming from for your web browser connection to work. It is also possible that the Gaia web portal daemon is dead, disabled, or running on an unexpected port, try the clish show web daemon-enable and show web ssl-port commands.
Attend my online "Be your Own TAC: Part Deux" CheckMates event
March 27th with sessions for both the EMEA and Americas time zones