I'm going to disagree.
30 minutes 4 times a week is generally not enough load to cause overtraining or warrant a break.
Secondly, taking a whole week off is not advisable. If you want a break, do a few easy spinning sessions or cut down to 1 ride/week. You don't want to go more than 5-6 days continuously off the bike.
I have days like the one you describe occasionally, I'm not sure what causes them, but it's not overtraining. Try again tomorrow, I'm pretty sure you'll do better.