Welcome to the forum @Ahmad1
I don't think your problem is related to the old thread that you have replied to. Please start a new thread. There please provide a lot more information.
What OS are you running node-red on?
Are you running the browser on the same computer as node-red is running?
When the site can't be reached is the the node-red server still running?
What URL are you using when it says it can't be reached?