Had too much to drink once and the combination of that and getting bored with fucking my g/f meant I couldn't get aroused enough to come. I penetrated her ok but lost interest really... faked an orgasm and went to sleep! (Of course now I've admitted I'm much more attracted to guys, I don't know what would have happened if it had been my b/f instead... probably would have been rock hard despite the booze!)
If I'm really tired I'll get a headache so that puts a stop to anything really, though it's not because I can't get hard, just that it makes my head hurt even worse!