agree with all the previous posts.
I would add to check to make sure you are in all the ranges for what you're eating (too much fat or too little protein can really zap my energy)
sometimes some new music on my MP3 is enough to re-energize me, or a new workout tee or shorts.
And if you've been going at it pretty hard for many days in a row, your body may just need a break. I'll go for weeks (sometimes months) without missing a day of working out. Some days are heavy cardio, some days are strength training, some days are just Pilates. As long as I feel like woking out, I do (often a morning and an afternoon workout). But I also let those two-a-days give me permission to skip a day (even 2!) if my body feels really tired.