power failure and a slow cooker
The important temperature would be what temperature it was at when the power was turned back on.

The official recommendations are to keep high risk foods out of the 'danger zone' of 40°F to 140°F for longer than 2hrs (cumulative).  If portions of the pot were at 140°F after being heated for an hour, and there was the time for it to cool down.  (and the time for it to have heated up initially), from a health department perspective, it may not be safe.

Personally, if it were me, I'd have turned it up to high to try to get it back up to ~200°F (about where most crock pot's 'low' setting is), and then depending on how close it was to done, either switched it back to low, or left it on high so it might be ready in time for dinner.  (however, I've also been known to eat raw beef and things that have been in the fridge for longer than the health folks recommend)