If you have been out all day and return to discover your power has been out, is it safe to eat your meal? Unfortunately, it is not. You have no idea how long it was without power, meaning unsafe temperatures could have caused bacteria to grow, and the last thing you want is the entire family to get salmonella or one of the other food bourne illnesses!
Additionally, most slow cookers will not automatically reset themselves after a loss of power, so even if the power was only out for 30 seconds, most slow cookers will reset and not automatically restart to the required temperature. Some older styles may reset, but you would need to know exactly how many minutes the slow cooker was not heating for.
The only time when it is safe to eat from a slow cooker when the power has gone out is when it goes out after the full cooking time is done that is recommended in the recipe – and you are home when the power goes out so you can be sure of the time the power went out. You can always use a meat thermometer to check the internal temperature of the meat you are cooking, so you can ensure it has met the proper temperature for fully cooked meat or poultry.