I have a tablespace which is offline. How do I bring it online?

Looks like one of the tablespaces has gone bad.

db2untag /dev/rudb52
db2untag: A service tool to remove the DB2 tag on a tablespace container.
The tag is used to prevent DB2 from reusing a container
in more than one tablespace.
If a tablespace/database is destroyed thru unnatural means,
then the tag can be left behind preventing future DB2 use of
the resource.
WARNING: This tool should only be used by informed sysadmins.
Using file </dev/rudb52>
ERROR: File does not exist.

Tablespaces go offline if there is an I/O problem. You seem to be missing the container; find out the container names that the the database expects, recreate them; restore the tablespace from a backup and roll forward that tablespace to the end of logs and then bring it online using "alter tablespace TBSP switch online". I do not know why you tried an untag. I use it in testdatabases for DMS tablespaces just before refreshing them with a newer production copy.

db2untag without the -f option tells whether there is any problem with a raw device. The raw device container in question shows that it does not exist. However, a list containers show that it is part of the tablespace. That's how I realized that the raw device is bad.

Thanks for the information.