What does it mean when a man is horny but cannot get his penis aroused?

This is not a one answer fits all question. This probably happens to most men at some time or another and is not necessarily a serious problem although it may be embarassing at the time. One of the most frequent reasons is that it happens once and he worries so much about it, that happens again. Too much alcohol is another frequent reason. Sex is as much mental as physical so the problem could be mental or physical. If it happens once or twice, don't worry about it. If it happens everytime or frequently, it's time to talk to the doctor. A physical and frank talk with the Doc may take of the problem. If it is something serious, he needs to be talking with the Doc anyway. Don't just do nothing and worry about it. There is nothing harder (pardon the pun)than getting hard, when you're worried about it. Most likely erectile dysfunction. Diabetes and the medication taken for high blood pressure can also affect erections. If you wake up with an erection it is probably mental rather than physical, and either way you need to see someone to get some help.