The "du" command will search through all the directories (optionally up to the requested depth) to check the disk space occupied by each file and directory. This can take a while, especially if you have a huge number of tiny files somewhere.
But if the du command really hangs forever, then there might be filesystem corruption and/or hard drive failure. Use "dmesg | less" to see a buffer of latest kernel messages: does the dmesg output contain anything that looks like a disk error?
The problem with the "dmesg" output is that it usually does not include timestamps for the messages. Usually, the important messages from dmesg are also stored to the syslog files in /var/log, which include a timestamp for each message. If you have a filesystem/disk problem, looking at the logs might allow you to determine when the problem started.
Note that if you have a filesystem/disk problem that causes processes to hang, any backups taken after the problem started might well be incomplete.