The first thing to check is to insure you didn't accidentally pop the t case or the overdrive into neutral.
If that is good, With the trans IN GEAR
and the cover off, SLOWLY
release the clutch and observe the output shaft (push it back on as soon as things start turning, or you will be wearing some 90WT, or you can drain it if you like). If the input shaft doesn't turn, you have an issue with the clutch. If it does turn, but the mainshaft doesn't, you have a problem with the input shaft, or the countershaft. If the output shaft turns, then....
Drop the inspection cover on the t case and let it drain. Reinstall the transmission cover and put it in gear. Have somebody slowly release the clutch while you observe to see if the transfer case gears are rotating. If not, you will probably have an issue with the planatary's in the O/D unit. This is not an uncommon issue and will require removing the O/D unit.
Don't forget that the boxes will be empty. You may want to pull a battery cable or incapacitate it to keep from accidentally driving it.