Muscle spasms

The house was pretty quiet today. We had a lot of things to do this afternoon. About an hour before we're leaving my husband was just sitting on a chair and he let out a screech. For no reason at all the muscles in his lower back completely went into spasms. His lower back was so tight it felt like concrete. We rushed to the pharmacy to find something for muscle relief but there is no over the counter muscle relaxer without a prescription. What a joke this is. We ended up just buying some aspracream. The aspracream only helps a very little so we got him on the heating pad, but its been four hours and he is still in pain. Any insight to this from anyone might be helpful for the next time.