Hey Buddy
This has happend to most of us at some point in time (if is hasn't happend to you it will do one day). I would say I have a high sex drive the wind has to blow the wrong way and I get a rock on, however I have had an occasion where I couldn't rise to the occasion and I had a ex page 3 model in bed with me & it was the first time we were getting giggy and all I could do was extend the amount of foreplay (Kissing,Oral Sex, Toys and fingering) she loved it and I got a second chance to show her what I got.
My advise to you is if this is a serious problem then seek advise from your GP as it could be a basic problem that can be fixed like diet, stress or lack of sleep or even excessive boozing. A thing to look into are Zinc losenges you can get them from you local health food store or failing that ask the doctor for that blue pill it always dose the trick
Hope that helps
Hot Chocolate