Situation 1.  The key fell off the Dell Vostro 1720 Keyboard and you lost the cap or the retainer. In this case you can purchase a single key here, just search for it. (Search examples: “Dell keyboard key”, “Toshiba keyboard key”, “HP keyboard key”, etc… )
After you found a new key, put it back in place the same way as I described in the situation 1.

Situation 2. The key cap and key retainer fell off the keyboard and you cannot figure out how to assemble them back.

It's all too easy to pop the keys off a laptop, and almost impossible to get them back on without losing or destroying the nigh-microscopic parts. This is a step-by-step diagram of how to put the keys back together on a Dell 4150 Inspiron, but the principles should work on other Dell laptops; these steps with HP Pavilion notebooks as well.

Don's be surprised, it is possible to replace individual keys on your laptop keyboard! Note that the same laptop model can have several different types of hinge mechanisms under the Dell Inspiron B130 keyboard key. Some colors in the pictures may differ for your model. That is fine, what matters most is the shape of the hinge piece.

One or more keys fell off the Dell Vostro 1720 Keyboard and you are not sure how to put them back? Don’t worry, most likely you can fix the keyboard and it’s not necessary to buy a new one. There is no universal repair guide because all keyboards are different. Here’s what I usually do in this case.

Step 1 Take a look at the laptop keyboard and see if the O-shaped clip that holds the key is still on the keyboard. This sits directly on the keyboard, beneath the key. If it is missing you will need to find a replacement and if it is still on the keyboard, you need to remove it by pulling it off.

Step 2 Slide the tabs of the U-shaped piece down and slide the O-shaped clip into the middle of the piece. The U-shaped piece clips into the keyboard and the O-shaped piece holds the bottom of the key. The larger O-clip should sit on an angle, with part of it raised up.

Step 3 Position the key directly above the two pieces and press on it until it clicks onto the proper spot. This gives you a functioning key, but you still need to put it back onto the laptop.

Step 4 Adjust the O-clip until you hear it click. This indicates that it is properly situated in the other piece and that they are locked together.

Step 5 Examine the area of the keyboard where the key was located before and look for the small holes that indicate the clip. Slip the edges of the clip into those holes and press down firmly until you hear it click. The key should now be working properly. You can repeat the steps as necessary for each additional key.