Pull the cowling and look at shift linkage. First question is;" Can you move the helm control forward and backward?"
Does the shift cable center rod move fwd and aft?
Is the shift cable hold down locked, broken? If it is the sheath of the cable will move but not the core.
If those simple tests don't tell you what the issue is, you will need to disconnect the shift cable at the motor from the shift lever and manually shift the lower unit. If you cant, the next step is to drop the lower unit and inspect the shift shaft to see if it moves ( in the mid section)
If it does then you need to see if the shift shaft in the lower unit works. Note that for theses tests of manually shifting, you may need to manually rotate the prop for the gears to mesh.