Put the t-case in neutral, then try shifting with the engine off. Try working the t-case shift lever from hi-neutral-low to jog the transmission gears a little. Some transmissions just don't like to shift worth a **** without things moving a little. If you can get it into gear with the engine off, warm it up & restart in first/low to load it. The stuck transmission/clutch bits may free up with a few minutes of use.
Might not be a bad clutch- I had a linkage bolt fall out, simulating a failed release mechanism.
Clutch may also be frozen to the flywheel (rusted throttle cable plus starting it in gear made for new shorts all around. 'Instant' oil pressure & 2900rpm + 8wheel gravel burnout.)
If you can start it in neutral (t-case too) try looking down thru the hole in the floor for the t-case lever. The output shaft should be slowly rotating. It'll speed up if you gently pull it into gear, even just enough for the synchro to touch. If you pull it into 2nd carefully, it should go with the engine running & t-case in neutral, even if the clutch is engaged.