I have loose tiles in my shower stalls in 2 areas. One areas is about 18" x 12", the other is just two tiles. Can this be repaired? Is this a relatively easy repair? Or this indicative of a major problem such as water damage and I need to start over with a new tile installation. Help!

Your question really cannot be answered until you remove the tile and see exactly what the condition is behind the tiles. There is no doubt that water has leaked behind the tile, but to what extent has damage occured? If the walls are sound then you can replace those tiles. If not, then the walls must come down. But you should act quickly since more time is more damage.

Thanks for the reply. How involved is the task of removing the tile? What should I look for in the backing the tile is adhered to? Most of all is this a DIY-fix someone that is a novice with tile?

It really is not that difficult, at times a little tricky.
Use a putty knife as close to the size of the tile. Insert it between the tile and the wall. Try to work it straight in without lifting the knife. Lifting may cause the tile to break. You can try tapping the knife lightly with a hammer to break it free from the adhesive. Sometimes a hair dryer blown on the tile will help soften the adhesive. Remember to keep one hand on the tile so that it does not drop into the tub and crack.
Once you have the tile off take a common screwdriver and probe gently into the drywall and notice the condition. If it is soft and crumbles then a wall replacement (or a section of it) will have to be replaced. If the wall seems solid then you can reinstall the tiles.
Apply the tile adhesive per the instructions on the can and allow the tile to set for one day. Replace the grout and allow this to set for a day. Ihope this helps and does not scare you off!!!!!