Andy L , May 13, 2010; 12:54 a.m.
Are the aperture blades visibly dirty or oily? Do they move at all or completely locked up? Does the lever move but not the blades or is the whole thing locked up?
When I started my first photography class I borrowed my mom's SRT and 58/1.4 and immediately overexposed everything. Oil from some other part of the lens had leaked onto the blades, making them stick. Common problem - any good repair guy can fix it. Most of the time this is what causes inoperative aperture.