I lost the screw of the SSD What can I do?

It is just a M2 screw. I think it is an M2x3, to be specific, though that may be off by a mm or so.

M2x3

M - Machine screw - this defines the thread pitch
2 - 2mm - this defines the screw diameter (not the screw head, which largely doesn't matter).
x
3 - 3mm - this defines the screw length, though this is the part I am guessing. It is okay to go shorter if you are unsure, but do not go too long, as screwing this screw in too far could damage your motherboard.

You can go to any specialty electronics store (i.e. radioshack) or even your local computer repair place, they will likely have replacement screws for a dollar or less that you can buy. If you are unsure on the length after looking at them, get multiple and use the one that fits the length best without being too long (you can check the length against the standoff they screw into; if it is longer than the standoff, DO NOT use it.)