2000 Mercury 125hp exlpto having problems reinstalling lower unit

Someone please help. I removed my lower unit to replace the Impeller. Everything went well till I went to replace it. The gears are not working properly and won’t shift. When I removed it it was in neutral. When I was reinstalling it, it wasn’t going on so I turned the flywheel and it worked. Since it seemed to be stuck In gear I removed it again to try and figure it out but no luck. While trying to figure it out I turned the gear shaft that is on the lower unit to see if that did anything but nothing. Hope I didn’t make this more complicated. Thanks in advance.

Second one today----Look at earlier post.----The lower unit jumped into forward when you were not looking.-----Put control in nuetral.----Put lower unit in nuetral.---Assemble.

And grease DS splines and shift shaft splines. I also put grease on the SS bolts and nuts to prevent seizing.

Thank you for helping me. Can you please tell how I can put the lower unit in neutral

Turn the LU shift shaft by hand counter-clockwise until it
stops, then with Vise Grips tight on the bottom of the shaft turn it a
few more degrees CCW until it clicks into a stop, this
should be neutral - check by turning propshaft --
prop should spin freely.
Put control in N and go ahead and attach LU to driveshaft housing some turning of the flywheel will help the DS splines align.
Do not shift into reverse unless you turn the prop at the same time.

Thank you so much. Gonna go try now

Worked perfectly thank you!!! Your a genius

You're welcome, years of experience has paid off!