We use a set of chemicals called cryoprotectants to freeze eukaryotic cells. Usually we use DMSO, dimethylsulfoxide, which causes the crystalline structure of ice to spread out a bit and prevent rupture of the cells. Tissue freezing is another animal. Because a tissue is a complex overlaid structure of cells, it's often quite difficult to preserve the structure due to a combination of uneven freezing, delicate connection, etc. Since structure almost always dictates function, you will lose some function as you disrupt structure, making it more difficult to derive information from your frozen bits.