Broken or Sticking Key
The most common repair call I get is for a "broken key." Usually, the customer means that when the key is pressed down, the note doesn't play. Or they may mean that the key is depressed and won't come back up. In either case, my response is the same. There are at least a dozen different reasons why a note won't play. Some of these may take me only a few seconds to fix, and some may take me half an hour or more. Most often, they are quick fixes - something is stuck, or something is broken but easily fixed. In that case, I do not charge anything if I am already tuning the piano. If it takes more than a few minutes to fix, I charge $40 an hour if I am already there tuning the piano, $45 an hour if I make a special trip for the repair.
In a newer piano, the most common cause of a sticking key is a felt bushing binding against the front rail pin (a pin that pokes up from the keybed into the bottom of the key). This is more likely to occur in humid weather, since the humidity causes the felt to swell up. I have a special tool which squeezes and compresses the felt. No more sticking key!
Sometimes the hammers are too close together and rub against each other, causing one note not to play. This is also easily adjusted in most pianos.
In older upright pianos, there are several likely causes of what appears to be a sticking key - none of which have to do with the key actually sticking. The jack flanges often come unglued. The key itself can crack at the balance rail hole. (This really is a "broken key.") The hammer shank can break. These repairs take more time. But I have experience in repairing them all, and many, many more.
And, of course, there are animal-induced and human-induced sticking keys I have encountered: mouse turds or loose change stuck between the keys. Pencils, toys, and other objects which have fallen into the piano and are lying on top of the backs of the keys.