Low batteries can affect the functionality of laptop keyboards via some indirect circumstances, and wireless desktop keyboards. For laptops, if you are running a full-screen application on relatively low battery, when the low battery warnings pop up you may not see them; at the same time, the keyboard may become unresponsive. To remedy this, plug the computer into a power supply or connect an external keyboard temporarily to dismiss the low battery alert. For Wireless keyboards, replacing low batteries should help fix the problem.

If you have spilled water or other fluids onto your keyboard, the keys will short out and not work correctly. In the case of sticky or salty drinks this can be a relatively permanent problem, but in the case of pure water, as long as it fully and quickly evaporates you should be able to regain function of your keyboard. Use a canned-air blower to blast water out from between the keys and let the keyboard sit to dry for a while. Putting it in relatively warm and dry places can help speed this up. Only use the keyboard after it has completely dried.

For keys that stick after sugary liquid has spilled, you should be able to pry them up by inserting a fingernail under the top edge of the key and gently prying up while pressing on the bottom. The key should snap up and swing open, exposing the bottom surface. Use a damp (not wet) Q-tip to carefully wipe away as much sticky mess as possible, and check the button mechanism to make sure it is functioning properly. Then place the keycap back on and press down to snap it in place.

Depending on the situation, sometimes specific key subsets--such as the Eject, Caps Lock, or the Function keys--may not work. This can sometimes happen randomly, or after running certain programs and especially other operating systems via Bootcamp. There are several things to check when this happens. First, in later-model keyboards and computers, Apple has delayed the activation of both the Caps Lock and Eject buttons to prevent accidental activation so that quick taps wont activate the keys. Additionally, for F-key functionality, ensure the F-key behavior is set according to your preferences in the "Keyboard & Mouse" system preferences. Under the "Keyboard" tab, check or uncheck the "Use all F1, F2, etc. keys as standard function keys" according to your preferences. If it is checked, the default features (Brightness, Expose, Volume, etc.) will only work if you simultaneously hold down the "Fn" key. Lastly, be sure Number Lock is not activated, because this will make standard keys output numbers. This can be toggled on some computers and keyboards by pressing Fn-F6.

Various system settings can also affect keyboard function. These are outlined by Apple in this knowledgebase article.

Hold the shift key while booting up to load only essential drivers.

Account-specific settings that are faulty can affect input devices, so try creating a fresh account in the "Accounts" system preferences to test the keyboard.

Without booting into Safe mode, try turning off services such as Bluetooth and Wi-Fi. You will need to attach a wired mouse to do this.

In addition to the previous suggestions, unplug all devices and turning off all wireless technologies.

When running Windows in Bootcamp, run Apple software update to ensure the latest Bootcamp drivers are installed. These can be downloaded from the following locations:

Firmware and low-level hardware problems can interfere with keyboard function. In some cases this can be attributed to faulty keyboard PRAM settings, but in other instances USB ports can stop working, which may be the result of faulty SMC settings. For information on when and how to reset the PRAM and SMC, please read this MacFixIt article Apple's built-in keyboards and mice are on their own USB busses, which in some laptop models is shared with one external USB port. In rare cases this can lead to interference with devices plugged into that port, so try removing USB devices. For desktops, trying different USB ports can also help track down the problem.

Beyond firmware settings, ensure your keyboard has the latest firmware installed. Apple has released firmware updates for laptops and specifically for Apple's aluminum keyboards, and these updates should be installed if you are experiencing problems with your keyboards. These should be available through Software Update, or specific updates for your computer can be found here: http://www.apple.com/downloads/macosx/apple/firmware_hardware/