Is this on a PC keyboard or a laptop? First, reboot your computer... (ironically this fixes most problems). Check to see if the problem persists on various applications, Word, Notepad, Excel, IE... If you press hard on the key, does it work then? Try, if a PC keyboard, unplugging the keyboard from the computer and plugging it back it.
Lastly, you can try removing the key itself with a flat-head screwdriver. You may have a dirt or something obstructing the key from making a connection to the keyboard. So removing the key, cleaning it with warm water and soap and then cleaning the connector with a damp (not dripping wet) towel.
There are probably YouTube videos that can demonstrate this.