When your penis is soft (flaccid) the smooth muscle in the corpus cavernosa is actually tensing and this squashes the spongy tissue so that blood can’t flow into it.
When you get turned on (aroused), your brain sends a message via the nervous system to the penis and various chemicals are released that cause the smooth muscles to relax. This allows blood to flow into the spongy tissue in the corpus cavernosa. The blood becomes trapped in these two cylinders as there is pressure on the veins that normally carry blood out of the penis. This is how an erection occurs.
From the above descriptions you will have noticed that there are a number of body systems involved in making the penis work. The nervous system has to send messages from the brain to the penis, the chemicals telling the smooth muscles to relax have to be released in the right quantities. The circulatory system has to be able to carry the blood into the penis. The spongy tissue in the penis has to be able to fill with blood. A problem with any one of these systems could cause erection problems.