I use a Sigma 28-80mm on a Canon Rebel EOS. Neither the zoom nor the focus rings will turn. Is there something that I could diagnose the problem? Can I fix this myself or will I have to send it in for repairs?
If the lens is stuck to the camera, you most likely have a distorted barrel. Most often this happens if the lens bumped against something while being attached to the camera. This can deform the lens mount and sometimes also some parts inside the lens. This can prevent the zoom/focus ring from turning smoothly. I had that issue with some heavier lenses over time where carrying the camera with the lens attached and sometimes bumping into things slowly bent some parts more and more out of place until it would stop working properly or it would become more difficult to get the lens off the camera. In some cases the contacts between the camera and the lens might not be aligned properly and while manual operation would still work, auto focus will be impaired.
If I understand you correctly you cannot get the lens off the camera and you cannot turn the focus and zoom rings. This would tell me that there is some distortion in your lens that bent the barrel far enough to cause this.
In general it is recommended to bring the camera and lens for service since the chance is high that you will cause further damage while trying to dislodge it - and then make the repair bill even higher.