Take it slow, and don't force anything.
How did you get it apart?
Usually (for me) it's the four posts at the bottom (beneath the keys) that are somewhat troublesome. I usually have to shave around the edges of the mushroomed post ends to make them fit back into the holes in the front.
Perhaps the same thing is happening up top? There should be four posts (two on either side of the battery compartment) on the front case, which feed into four holes on the back (underneath the clear red battery cover). If the ends of those are still mushroomed a bit, you may wish to shave them narrower. (I wouldn't shorten them so much as make 'em skinnier -- the idea is to leave them just wide enough that they fit into the holes, but still maintain a frictional bond to keep the calculator held together.)
There are also several small guide pins (the number varied with various slightly different versions of the Pioneer case) about the edges of the front which fit into "tubes" in the back case. If any of those are bent, that can prevent close alignment of the case halves.
Take a breath, work slowly, and try to see what the hold-up is. Good luck, and post again if you continue to have trouble.
