That's called blocked eustachian tube.
The eustachian tubes help the ears drain fluid and they keep the pressure in ears in check.
It often gets better on its own after a while, but you can try closing your mouth, holding your nose, and gently blowing (similarly like you would if you were blowing your nose). Chewing gum or yawning can help.
Some people have reported good success using
EarPopper.
For cases that you can't resolve on your own, a doctor can insert ear tubes, but a better alternative for chronic eustachian tube dysfunction is a balloon.
Balloon Offers Relief from Chronic Eustachian Tube Dysfunction