If you accidentally swallow a bit of food into your lungs somehow then you'll usually begin coughing right away and may feel like you can't breathe.
Just keep coughing and usually the food will work it's way out of your lungs and nothing bad will happen.
Take something to drink some was a glass of water and after you stop coughing drink the water to help push the food down your throat and into your stomach.
Some other signs of food swallowed in your lungs are pain when swallowing, heartburn or chest discomfort, trouble starting to swallow, coughing or wheezing when trying to eat.
If you constantly get food swallowed into your lungs or are always coughing when eating you should see a doctor because something could be wrong with your throat.
Normally food should go down your throat and not your lungs but sometimes accidentally the food may go into your lungs but not a lot should.