Excellent post Ronald! Basically...if it happens to me, just ask the guy what to do! Sometimes it's nerves one way or another, (fear of getting caught, fear of failing at sex, fear of looking weird, ect) or if you are in the middle of foreplay...it might just be tired :P I know sometimes if My Fiancee and I extend our foreplay too long, It will...take a break even though i am really really REALLY turned on emotionally.
Sometimes the best thing to do for a guy is the best thing to do for a girl in this case. Tell him to relax, breath, and just be really sexy. Slowly turn him back on. Go back to foreplay for a bit.
If it really IS something more serious then just tired/worn out, then you need to talk to him and address it, just like if a girl were having issues in the bedroom, guys can have issues as well. you may need to strangle it out of us, but We will tell you
