I have been using a Canon 60D for five years and recently, only when I am using my Sigma 24-70mm lens, it does not let me take pictures. When I look into the viewfinder, it is so dark (I assume that the lens is not capturing light). And the camera either does not respond (i.e., when I press the shutter button to take a picture, it does not respond) or even if it does, the pictures turns out to be very dark. This behaviour is so abnormal and totally random. One minute it is working fine and the next moment it goes nuts. I have tried re-mounting my lens but it still does not make a difference.
I also have a 50 mm lens but my camera works fine when I use that.
Can somebody please help me understand what's the issue and how to fix it? I am a total noob when it comes to photography and this issue is really making me anxious.