What's wrong with my Sigma 18-250mm lens?

Seems that your ISO value in the camera is set to some fixed low value.

Set ISO to "Auto" or, if you prefer to work with a fixed ISO setting, set it to some high value, e.g. 6400 and then experiment- successively set it to lower value.

It sounds to me like the aperture control in the lens has failed and it will need to be serviced.

All Auto Focus camera lenses are wide open until you actually take the photo. Look though the front or rear of the lens, and it should be completely open. If you can see the hexagon type shape of the aperture blades, it means they are not working properly. You will not be able to auto focus and the viewfinder will be very dark.