Under certain circumstances it might happen that rpm,a nd therefore yum, doesn’t continue to work. The program will simply not continue any action and also cannot be interrupted any more.
strace the problem you will see a string like
This is a complicated bug known for years and it will hopefully be fixed at some time in the future. However, until then the best workaround is to quit all running yum/rpm instances and delete some files:
# rm /var/lib/rpm/__db*
A reboot might also help, but this is definitely much faster.