Please complete the following initial trouble-shooting steps to force the license information to be re-read.
a. Log into the Tableau Server host machine as a local administrator.
b. Open the Command Prompt as an administrator.
c. Change directories to the "bin" folder in the Tableau Server directory.
d. Type the following commands:
e. Please have a user log into Tableau Server verify that the "license expiration" message no longer appears.
Paul has provided some very useful steps for Server, which I would advise following.
As a side note - you can view your current licenses via the customer portal.
Log in - go to My Keys
You should see something like the below -
Maybe useful to double check