The thread you originally posted this to was for a different line of appliances where the offered solutions don't apply.
I forked your comment and created a new thread in the right area.
If you cannot access via WebUI or SSH, your only option is to access via Serial Console.
Can you log in via Serial Console?
What errors/messages do you see on the serial console?
Otherwise, your only option is a factory reset using the recessed button on the back of the appliance (though beware of: https://support.checkpoint.com/results/sk/sk123499).
Note that the 600 Series of appliances are End of Life.
You can download the latest firmware for this appliance from here: https://support.checkpoint.com/results/sk/sk123294
Note: this requires a valid support agreement.