All pictures come out too dark now when they used to come out fine.

My camera is a couple years old. I’ve been able to take good pictures with it for a long time but now all the pictures come out super dark, but not completely black. The only time the picture came out okay was in the middle of a sunny day in the snow. I checked to see if the shutter opens and I do see it open without film in the camera but I’m not sure if it’s opening completely or just partially. I am also not sure if the film went bad. I had kept it in the car for a while which can get pretty hot, but I tried with multiple packages and the same result. The film does not expire for a couple more months. I never had this problem before and I don’t know what to do about it now. I don’t know if it’s worth it to send it in or just buy a new one at this point.