Dell Desktop (XPS 8500) wont' shut down

I have a Dell XPS 8500 and I am having issues with the PC when it comes to it shutting down. There were a few cases where I would turn off the PC, and the monitor and operating system would shut down, however the power light would remain on, and the PC itself would remain on. I ended up having to manually press the power button for a few seconds to force it to close. However, I know this is not good for the PC and at the same time, I want to find out what is causing this problem. If this happened only once or twice, I wouldn't' mind, but this has happened a few times already, so I want to know what is happening with my PC.Any kind of advice would be appreciated.