Could have been a mechanical failure, but if they were doing the proper preventative maintenance it should have been caught.
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Yep, it sounds like someone did not do something right, maintenance or otherwise. Especially if it took that long to get back up and running -- I was on a LORAN navigational transmitting station while in the USCG. We had backup generators, clunkers by today's standards, and we never had a problem firing up those in very short order to keep our signal on the air. Of course, we took very good care of them --- oh well.
I've been a roving cellular engineer for about 20 years now. We've installed hundreds of generators like the one shown in the photo at cellular switching offices and transmission sites Before going into wireless, I worked for a major fire alarm manufacturer and we routinely interfaced the fire alarm into the generator control so the line power would automatically disconnect and transfer to the generator if the alarm went off. These type systems are common in high rise office towers, hospitals, etc. It was either mechanical failure or somebody didn't do their PM work......pencil whipping preventative maintenance will bite you in the arse every time....