Automatic or stick? If an auto another possible cause, the shift linkage bushings. If worn enough, they can cause the shift lever to disengage the safety switch. Result, nothing, no sound no click no anything when you turn the key. When you went out to try after an hour, maybe you had bumped the shift lever enough to engage, resulting in a start. If it happens again, try lifting/pushing the lever all the way to park and hold it there while turning the key.
73 280 SEL 4.5