There is a very good article on moving Oracle data files to be found here: https://docs.oracle.com/cd/B28359_01/server.111/b28310/dfiles005.htm
Unfortunately, it does no include a final step to bring the tablespace back online.
The command is simple:
ALTER TABLESPACE USERS
So, execute the procedure in the link above, and when finished, make sure you bring your table-space back online.