If you can't use the flash, and the concert is indoors (ie, dimly lit), your main problem is going to be motion blur with the slow shutter speeds that will be required to properly expose the image. You may want to buy, borrow, or improvise a tripod. If you have some flexibility in where you can place yourself, you can brace the camera against any immobile object, so that the camera itself doesn't move at all while the shutter is open.
Before hand, experiment shooting indoors without any flash, in AV mode at F2, with ISO 200 and 400, to see what the results are like. 400 will give you twice as fast a shutter, but a lot more noise. You can remove much of that noise later in software (plenty of threads here on that subject).
You might be able to get some neat, creative effects with mild motion blur in the musicians--you just want to be sure none of it is coming from motion of the camera itself.