First check your lens iris blades are working press on the depth of field lever on the lens to close the iris blades, look carefully do you see any shiny or wet marks on the blades looking in from both sides. Also does the blades close with a snap or do they appear sluggish? In either case the lens needs service as the grease that is on the focusing helical unit has migrated to the blades gumming them up. This often happens when you expose your equipment in a high temperatures.
If the lens appears OK then the AE unit within the camera needs repairs. There is a small electro-magnet that can get gummed up as well, so if fails to release.
My camera had both problems. In either case I wouldn't recommend doing either repair your self. Both are quite tricky to fix and require special tools.