I had the same problem with the Wahl Pro Series Rechargeable model 8552, and called Wahl service. Here is what I learned:
Never unscrew the screws on a Wahl blade (they use those funky nonstandard screws to deter you) -- that's the main thing. The whole blade assembly is supposed to come off by pulling it up and off from the teeth.
But, if you did it (as I did), you can put it back together this way:
1. Place the large blade down on the table (Wahl logo down), and place the small blade on top of it (if you look carefully, you will see how it lines up).
2. Insert the spring points into the two holes of the white plastic part (on mine, the spring kept slipping out and I finally used a drop of superglue on each hole until it would stay in long enough to complete the assembly). Place that piece on top of the small blade (see photo).
3. The bent metal piece with the two screw holes then gets placed on top of the large blade, with the center ridge of the bent piece over the straight center portion of the spring -- so that you have to hold it in your hand and push it together against the spring in order to align the holes of the entire thing. As you are holding that, put the screws in from below and tighten. It's awkward and if you are like me, it will take several tries before you can get it all lined up correctly and tightened. The photo below is what the final assembly looks like before snapping it onto the clipper.
3. Clip it in: At the top of the assembly in the photo below, notice the small bent metal piece that will fit into the back of your clipper in the hole. Once you fit that properly, push the whole assembly down onto the clipper until it snaps into place.