@turquoise-- Don't assume that you have a herniated disc, get an MRI to know for sure.
When a spinal disc is herniated, spinal fluid inside the disc leaks out and applies pressure on the surrounding nerves. If the pressure is too much, it can prevent the nerve from sending signals to surrounding nerves. This is what causes numbness in the back.
It's actually very dangerous because if the nerves become damaged, it can even lead to partial paralysis.
It is usually treated with muscle-relaxers, pain relievers and rest to relieve the pressure on the nerves. In very serious cases, surgery may be necessary.