I was taking some pictures on my camera a Nikon-D3100 when I noticed a black line appearing at the top of the photos, doing a bit of research I thought it might be a problem with the shutter, could someone help me confirm if that's the problem? As you an see I am not a professional photographer I would really appreciate any help
That result is characteristic of using a shutter speed that is too fast with flash, and manual mode will allow you to do this. The flash sync speed for the D3100 is 1/200 and no faster. If the flash is off camera you may have to use a slower shutter speed than max x-sync in order to allow for some communication delay (but the exposed part looks like on-camera flash, so probably not an issue).
Edit: given the latest comment there is a physical obstruction in the way of the sensor. Take the lens off and I suspect you will find a stuck/broken shutter curtain.