I'm using an Fortgate 50b router, but without knowing why, I lost the configuration of the gatway.
How do I reset to the initial configuration?
I can't access the web-config on https://192.168.1.99.
Best bet would be to get the console cable and use a terminal program such as Putty, Hyperterminal or TeraTerm on a serial port. You can get cli access to the Fortigate that way and if you've still got the username and password, log int and then type in (as almost suggested above) execute factory-reset. That should drop you back to the default settings. If you don't know the username and password (defaults are just 'admin' with no password), you can try the following:
(blog is nothing to do with me but no point copying and pasting its content here)
Or if that doesn't work you'll need to re-upload the firmware via TFTP by breaking the boot sequence. You'll need access to the firmware downloads from Fortinet though which would require a valid support contract.
http://kb.fortinet.com may also be your friend here.
Check to see if it is still sending out DHCP. Get the gateway address from it and use that to connect to it.
To reset the FortiGate unit to the factory defaults, in the CLI type the command
You might also want to read the QuickStart Guide