I have never had erection problems, but yesterday while my wife and I were having sex I could not get it up. Should I take some medicine?

Not yet. This is likely to have been a one-off experience. Please note that almost all men face erection problems at least once in their lives. This doesn't imply that they have erectile dysfunction. So worry not, for now. If it becomes a pattern, that's when medical intervention is required.  And please erase this incident from your mind, for anxiety can cause erection problems too.