clutch replacement gear shifter was hard to move

Replaced the the clutch set in my truck. I believe the transmission was in neutral when we pulled it out. We did not drain the fluid, like we should have, so we did leak fluid after removing the drive shaft. We turned the transmission on it’s side and completely removed it, fluid also leaked from the gear shifter when it was being removed. After successfully replacing the clutch and putting the transmission back in and replacing the fluid (ATF), notices the gear shifter was hard to move, appears to be in the neutral position, but it’s actually stuck in first. Drove it around just to see if it would unstick itself from the gear it’s in, but no luck. What could have happened? Any help would be appreciated.