The Shuttle can't fly much higher than 300 miles. It simply doesn't have the delta-v necessary to reach higher altitudes. It would be possible to build an robotic (or even manned) vehicle that could go out to a LaGrange point to perform the repair but none is in the works. Without a real need for such a vehicle, it would be cost prohibitive to build one just for NGST. You'd also have to design NGST to be serviced. From everything I've read about the project, that isn't the case. As a result, it'll have to be built more robustly than the Hubble was because once launched, it can't be fixed.