Monday! Shit!

So another week has rolled around and for me that means back to the start of my workout plan. Today is Rest day as all Mondays should be, then its upper body tomorrow, but that's not what I want to talk to you about, Yesterday was cardio day which usually means a forty min kettle bell workout but I couldn't manage it, after twenty minutes I was wreaked and I had a splitting headache, which proceeded to get worse as the day went on.  Normally I have no problem motivating myself but i just couldn't get going.  So to my question, how do people motivate themselves? What do you do to maintain the drive to get your lazy ass out of bed and workout?