Help ! rpm or yum freezes

Sometimes, rpm, yum or even other commands are freezing without message to understand why (e.g. for yum, just after the message "Running Transaction Test").

Almost each time I've seen that, it was caused by broken mount (usually network ones, when the server at the other side has failed, or has restarted).

To check it:
mount

If you see some network mount points you do not need anymore, or you're not sure about, umount them.
If they are broken or if the system wrongly thinks it is busy you can use the -l option (for lazy) to ensure the umount - in fact the system won't fully perform the umount but it will clean the list of mounted point, such a way the launched commands won't freeze anymore.

For instance, to umount (lazy) a broken mount point /mnt/myMountPoint:

mount -l /mnt/myMountPoint