My CPU is running but the monitor won’t display anything

This was problem that I faced few days back. A friend of mine called me and told me that her monitor won’t display anything and asked me to come and fix the computer. I went to her house and was pretty amazed to see that CPU was running, the monitor was also getting the power (the power light of the monitor was blinking) but it wasn’t displaying anything. I didn’t know what to do. I checked the connection from the monitor to the CPU and it was also fine. Then, I opened the casing to check if something was wrong inside. By the way, I am not that good at hardware stuffs but still … just to check. There were two RAMs and 2 hard-drives connected. Then, I just plucked out one of the memory (RAM) and restarted the computer. The computer booted normally. Then, I thought that may be the memory or the memory slot was damaged. Then, I cleaned the memory and the slot with a cloth and put the memory back the slot and turned on the power. The computer booted fine and still is. I was happy and my friend was too because her problem was solved but I was not satisfied. I couldn’t find out what the real cause was? Was it really the memory, the memory slot or some other cause?