1) Are you trying to set up fresh tableau server? If yes, please list the steps that you have completed and also if the tableau server has been installed on cloud. If not, then what has changed since the last time you were able to successfully log in to the tableau server via the web link?
2) Is the tableau server behind a load balancer? If yes, then verify that the load balancer is able to reach to all the nodes in the cluster (provided you have set up a cluster)
3) In the tableau server configuration tool - have you enabled SSL? If yes, then verify the keys are valid.
Also, make sure that the tableau server is up and running.