Have a look at any C / E / S / SL shop manual from that era. There is a 4 or 5 mm allen bolt near or behind the weather strip. Unscrew this and the lock cylinder / rectangular trim will pull out of the door. ( I'm pretty sure the bolt stays captive in the door ) I'm thinking the end of the cylinder broke off. The end is a shaft and a flattened tab.

If this isn't the problem, look through the hole for broken linkage. If that does not turn up any broken parts, the interior door panel must come off.