Dan,

If you are getting zero boost now and it was boosting just fine the day before ... you may have a simple problem. The rod that extends from the wastegate actuator to the wastegate may have slipped off. The actuator is connected to the underside of the turbo. It has a vacuum (boost) line connected to it that runs from the intake manifold. When the boost reaches a certain level the actuator acts ...pushing the rod out and opening the wastegate. This dumps exhaust pressure before it can spin the turbo blades. If the actuator rod has slipped off the wastgate the gate is open all the time and you will have little, if any boost. The rod is held in place with a clip that runs through a small hole in the arm. These can break and fall out allowing the rod to slip off the wastegate and the gate to open. It resutls in an immediate lose of boost. Usually a turbo fails over time and with some obvious symptons. Check the actuator rod and sometimes the bolt holding the actuator in place comes loose. Check that as well because even with the rod in place a loose actuator body causes the rod to extend and also opens the gate. Either one of these problems is quickly and easily corrected. Check the actuator and rod first before fretting too much about the health of the turbo.
Holes/leaks in the in take plumbing and a clogged cat can also causes a sudden drop in boost. Check those things next if the actuator/rod is OK.
Hope this helps and your problem is a simple one.