Assuming that you have not spilled any liquids on the keyboard or damaged it in any way, verify the purchase date on the sales receipt or purchase document for the laptop and if it is less than 12 months old, consult the warranty statement in the documentation that came with the laptop, as to what to do to make a claim for a warrant repair or replacement.
If you cannot find the documentation, go online and search for Acer Aspire One D257 warranty for your location.
If it is no longer in warranty, initially plug in a USB keyboard to at least still be able to use the laptop.
Here is a link to just one video on how to replace the keyboard on your laptop, (there are others just search for Acer Aspire One D257 keyboard replacement)
Unless it is a loose keyboard cable connection on the motherboard, it is better to replace the keyboard than try to fix it. You can cause more problems than you had before you started if you try to fix it.
Replacement keyboards are available on places such as Ebay.
Here is a link. It is not a recommendation to use the sellers. It is shown to give an idea of the price of the part.
Hopefully this is of some help.