Turn the key upside down to make sure the inside of the key is not damaged. It's possible that the key became detached because of damage.  The key should have a plastic piece inside that connects back onto the plunger that is on the base of the keyboard. If your key is damaged, then you will need to find a replacement from an old keyboard or an online retailer that offers replacement keys. Look at the square space where the key will fit back onto the keyboard. In the center of the base, you should see a small, tube-like plunger that will attach to the key. This is the piece that moves up and down as you type. It should be in place and unbroken.  Each key has its own plunger. When you press down on the key, the plunger registers that you are pressing down that letter. If the plunger is broken or missing, you will need to replace your entire keyboard because the key cannot be repaired. Hold your key over the plunger. Get down low and turn your head to the side so that you can see the key fitting into the base. Make sure that the key is in line with the plunger. It should fit evenly into the base. Gently press the key back into its place on the keyboard. Don't press too hard because you could damage the plunger. When the key is secure, you should hear a click. Push the button several times to make sure that the key works. It should move smoothly up and down. If it doesn't move smoothly, detach the key and try to line it up again. It's possible that the key or the plunger is damaged. If you cannot get the key to move smoothly after several attempts, then you may have to replace the keyboard.

Summary:
Look under the key to make sure it is intact. Check to make sure the plunger is intact. Align the key with the base and plunger. Press the key onto the plunger and wait for a click. Test to make sure the key works.