It happens to everyone. Maybe not work, but missing an important appointment. If you're stressed and really focused on something else, sometimes it's hard not to lose yourself a bit.
This past semester I was supposed to meet with one of my profs for a final evaluation/review at the end of the semester (it was a clinical class, so he wanted to get our feedback on the placements and whatnot). Anyway, I wrote it in my agenda (which is my lifeline to what I have to do) and carried on. The day before the meeting he emailed me and asked if we were supposed to meet TODAY? I was a little freaked, but told him I thought it was tomorrow. He realised it was the next day and all was well.
Until the next day when I was working on a paper and the meeting COMPLETELY escaped my mind. We rescheduled, it was all good. But let me tell you did I feel like an ***.
Just be honest. Explain. Apologize and remind them it has never happened before and won't happen again (you hope). Don't say that last part!