Photinia are large shrubs growing to 8 foot in hight, or more and 5-6 feet across. I have one which is beautiful here in Cheshire. They can, with care, be grown in large containers but are better off in soil. By the sounds of your question it appears that the pot they are in froze solid for a time last winter/spring with the very cold spell we all had and this has killed them.
However, look for any sign of activity at the bottom of the main stem by (as suggested by Verdun) scrape away some of the bark and if brown, it is dead but if green, there is still life in the plant. If so, remove the pot with the shrub to somewhere protected from your weather and cover it in fleece AND PRAY!!