Like the stock market and foreign car engines, erections are mysterious things that seem to have a mind of their own. When they don't happen, it can be disappointing, at the very least.
Decreased blood flow, typically because vessels that supply blood to the penis have narrowed, is often the cause of erectile dysfunction (ED) in older men. Emotional issues are more commonly at the root of it for younger men.
Regardless of the reasons, it's important to look for answers.