Dave is correct; the only food available 24 hours is room service.
You can get snacks and desserts at the buffet until around 12:00 or 1:00 a.m.; and continental breakfast starts at 6:00 a.m.. But for that five hours, room service is it.
Even lines that tout 24 hour buffets, like Princess, only have snacks. You are better off with room service--it's fresher.
As for the food not being that great--everyone's taste is different. I recently cruised HAL and thought the food was terrific.