i removed my down arrow key and can't put it back

You will need to figure out how to fit the two parts of the scissor mechanism back together which will most likely involve taking another arrow key or function key cap off so you can see how the two pieces of the scissor mechanism are meant to fit together.

Once the two white pieces of the scissor mechanism are together as one piece, then you will need to properly reattach it to the metal brackets in the key bay.

Finally, assuming you did not damage the clips on the underside of the key cap....you will want to slide the one side of the key cap onto the studs of the scissor mechanism, then pressing down on the other side of the key cap to engage the clips. If you look very closely at the underside of the key cap, you will notice on one side are two fragile clips and on the other side two protrusions which are meant to the the studs/protrusions of the scissor mechanism slide inside.