To echo Bruce, as a long distance runner I find that occasionally I just can't do it, or sometimes great sex just dosen't cut it. Sometimes it's a subtle warning that I'm coming down with something, or that I'm actually mentally exhausted and haven't got round to noticing, sometimes you just pick these things up for the first time in 'extreme' states - I find. If nothing else has changed in your desires, your heart or your mind then I'm sure it will turn out to be a 'glitch' and you'll be getting on with things again in no time. Maybe you're a bit run down or something? Don't beat yourself up about it though