We usually prefer to use laptops rather than desk computers, because laptops are convenient all-in-one computing devices, but their parts are often more difficult to replace than their desktop counterparts. The integrated keyboards of laptops, for instance, are troublesome to replace--you can't simply buy a new keyboard and plug it in. If a key falls off of a laptop, it is best to repair the key rather than get a new keyboard.
Prepare the tools which could greatly help your work, such as tweezers, superglue etc.
First of all, make sure you have all the parts that fell off. A Dell Inspiron 1545 Keyboard key consists of the key cap, which is the part of the key you press down, and a key retainer, which are two small interlocking plastic pieces underneath the key cap. Make sure to put all these small items together without any missing.
Secondly, Reinstall the key retainer without the key cap. If your key fell off the the key retainer still attached to the back of the key cap, you must remove the key retainer from the key cap. It should snap out without too much force. Then use your tweezers to snap the key retainer into the metal housings on the keyboard. To get an idea of how the key retainer fits into your particular keyboard, you can remove a different key to compare. (Removing a key by gently put firmly pulling it back from the top should just pop off the key cap, leaving the retainer in place) Usually installing the retainer involves slipping one end under some small metal hooks and snapping the other end in place.
Following, snap the key cap back onto the retainer. Once the retainer is in place, the key cap should snap back in place with a little downward pressure. If the retainer did not pop off initially, you may only have to snap the key cap back in place to fix the key.
Finally replace a damaged key cap or retainer with that of another key. If your key cap or retainer broke when it popped out, you can still repair the key by trading it for a lesser used key on your keyboard. For instance, if the retainer on your "A" key broke, you probably couldn't live without the A key, but you might be able to live without odd keys like the windows button or the key to the right of the right alt key. So simply remove the unused key's retainer and use in in place of the broken A key retainer. You can also consider using superglue to mend the broken key retainer, though this might cause the key to strike sightly differently than normal or be more prone to future breakage.
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