The first is when the cable box loses AC power or gets rebooted. The guide then needs to be downloaded again and it can take 24 hours or more depending on if the whole area lost power and all the boxes in the area need guide downloading.
The second reason is you having signal issues on the narrowband channel the guide information is carried on. This normally would need a tech visit to correct.
Also, make sure you are not routing your coax through any AC surge protector strip coax ports.