You've run out of heat sinks. Mass Effect 2 uses an ammo replacement called heatsinks that are universal to all weapons except heavy weapons. You can find them on the ground and on pieces of cover in battle areas. They are small red cylinder shaped pickups. Heavy Weapons have a special class of Ammo (the pickups are called Energy Cells) and if you have none the weapons will not fire. Another thing I have noticed, the game has a 'smart trigger, if you are behind an object that 'could' be used as cover and have no direct line of sight to a target you will not be able to fire. And finally, a limited number of Heavy Weapons have what is called a "Charge-Up" timer so tapping trigger will not fire them you have to press & hold till they fire.