If you feel normal clutch pedal resistance, but the clutch does not seem to be releasing and you can't get the transmission into gear with the engine running, try putting the shift lever into gear, depress the clutch pedal, and start the engine.
You could have a clutch disk that has attached to the flywheel or pressure plate. Sometimes condensation will make one or both surfaces rust a bit, and hang onto the clutch disk. It seems the clutch is not working, but it actually is releasing. There's enough 'sticky' to the disk that it will spin the gears in the transmission and prevent selecting a gear. Start in gear, and you'll hear a slight 'bang' as the clutch disk breaks free. It will then likely work fine until the next time it has sat for a while with rain or condensation, and it may re-cur. Had it happen with less than 4k miles on the truck, but figured it out and have known the 'fix' since.
WARNING: If you try to start in gear, MAKE SURE IT IS CLEAR in case the clutch does not release.
WARNING do at your own risk, more or less.
tom