If you just lifted off the top tree and it won't go back down on, this could mean a couple things. The tubes are bent a bit, the lower tree is bent a bit.
Either of these require you remove the tubes from the trees to check the tube straightness. To check the lower tree remove the forks, clamp a straight edge across one side of the bottom of the tree to see if it lines up with the other side. Both sides should set flat on the straight edge.
Rolling the forks on a flat table top with the lowers off the table and watching the lowers as you roll the forks can give you some idea of how straight they are. Better to tear down the forks and roll just the tubes.
Another possibility is that the tubes are twisted in the trees. Loosen the lower tree clamp bolts. Wiggle the tubes around till the upper drops on. You may have to loosen the fender mount bolts and axle too.
Once the top tree is on, start At the bottom of the forks and snug things up working up to the top.
Leo