The temperature of food stored in hot holding has dropped into the temperature danger zone for a period of two hours. What should be done?

Hot food should be held at an internal temperature of at least 135°F while it's waiting to be served to customers. An example of this is a buffet line. Theoretically, food could be held at this temperature (or higher) forever and still be safe to eat. But most establishments will throw food out at the end of the day if it's been hot-held all day. This is for quality reasons and not for safety. For safety, hot food should never fall below 135°F for more than 4 hours and if it does, it should be thrown out.
Your establishment should have policies for how often the internal temperature of hot-held food is taken. If the policy is that the temperature is taken every 4 hours and if the temperature falls below 135°F, you need to throw it out. This is because it could have been below 135°F for the entire 4 hours. However, if your establishment has the policy to check the temperature every 2 hours and it falls below, then corrective action can be taken. The food can be quickly heated back up to 135°F and still be safe to eat.