Crock pot left on high for longer than recipe called for
Make sure nothing burned onto the sides (scrape it out if necessary), and add extra water if too much has boiled off.

After that, I'd probably check it after 3 hours on low, pretending that the hour on high cooked it twice as much as it would have on low, and see if you think it's done. No harm letting it go the full remaining 4-5 hours if it seems like it needs it, or like it won't hurt.

The recipe probably isn't very sensitive anyway; there's a good chance even if you had followed it exactly you'd think it was fine over a pretty broad time range.

...all that said, a chicken cooked on high for one hour and low for one hour sounds like it'd probably be done to me. I'm not sure why the cooking time is so long, and you could honestly just check it right now.