'''Keys are Stuck'''
There might be crumbs stuck inside your keyboard. Try holding your laptop upside down or at a 45 degree angle from being horizontal and give it a few good shakes that will dislodge anything stuck between your keys. Use a keyboard vacuum or a can of compressed air, these can help to remove any particles in between the keys interfering with its usability.
'''Loose Cable'''
The ribbon cable connected to the keyboard may have come loose. Fix this by reconnecting the cable.
'''Water Damage'''
If you spilled any liquids, especially soda that can get sticky when dried, apply some cleaning solution, such as rubbing alcohol or soapy water, to a dish towel and gently rub down the affected keys. You can also remove keys and wipe it from the inside.
If those methods are not applicable, try rebooting your computer. If none of these work, try replacing the keyboard.