The most common cause is a stuck key, either from whacking it too hard, or something sticky getting underneath it.
Run one finger across each row to see if a key is lower than the rest. Push any such key gently and move slightly side to side to see if it will unstick.
If it has just wedged from being used too roughly, it will often pop back up quickly, if it is sticky underneath, it will probably stick down again as soon as you push it again.
For this issue, assuming you have a spare keyboard you can use for a few days, stick the dirty keyboard in the dishwasher on a cool cycle, then give it a good shake to get the excess water out, and leave to dry thoroughly for a few days, somewhere warm.