Yeow! Normal readings are 70-120, with 100 being ideal. Above 140 is high. Below 70 is low. Below 60 is edging into danger. I was having a blood sugar crash. That's the lowest blood sugar reading I've ever clocked.
Fortunately, I'm usually prepared for these sort of things. To some extent, it's a cause for minor celebration, because it meant I could have orange juice (I normally avoid it because it spikes blood sugar high), and I ate one of the food bars I keep in my computer bag for times like this. After maybe twenty minutes or so, I started feeling more or less normal again.
Thank goodness I recognized the symptoms before they got worse. It probably wouldn't have been really dangerous (as a Type II diabetic, I have to worry about highs more than lows), but it was still unpleasant.