The SWITCH ONLINE clause of the ALTER TABLESPACE statement can be used to remove the OFFLINE state from a table space if the containers associated with that table space have become accessible.
The table space has the OFFLINE state removed while the rest of the database is still up and being used.
An alternative to the use of this clause is to disconnect all applications from the database and then to have the applications connect to the database again. This removes the OFFLINE state from the table space.
db2 ALTER TABLESPACE <name>
SWITCH ONLINE