This may be a dumb question, but with all the pitot tubes I see hanging on the nose of every commercial Boeing & Airbus jetliner I have had the pleasure of boarding.. how does such a (triple, IIRC) redundant system fail so completely?
The only dumb question is the one you don't ask!
I can think of several instances where total Pitot/Static System Failure has occurred for various reasons. To name a few:
-Bergenair 301, a Boeing 757-225 that crashed in February 1996 off the coast of the Dominican Republic due to a single pitot tube blocked by a mud dauber wasp nest.
-Aeroperu 603, a Boeing 757-23A that crashed in October 1996 after departing Lima, the cause was determined to be tape left on the static port after the aircraft was washed.
-Air France 477, an Airbus A330-203 that crashed into the Atlantic in June 2009 off the coast of Brazil. The cause of this crash has many factors, but the sequence of events started with inconsistencies in airspeed readings, likely due to the aircraft's pitot tubes being blocked by ice crystals after the heat system malfunctioned.
So, to answer your question, sometimes it's nature, sometimes it's human error, sometimes it's mechanical failure. A "lost" instrument reading my not be "lost" all together. If the Captain's side is reading different then the F.O.'s, they may not know what is the true reading, if either of them.
According to The Cambridge Aerospace Dictionary, the definition is as follows:"PITOT/STATIC SYSTEM: INSTRUMENTATION SYSTEM FED BY COMBINATION OF PITOT PRESSURE AND LOCAL STATIC PRESSURE, DIFFERENCE GIVING DYNAMIC HEAD AND THUS ASIR (air speed indicator reading)."
In other words, the system relies on the difference between the two to come up with the data the instrument is to display, and when one part of the equation is missing, it can and will display incorrect information to the point of being deadly.
Hope this helps...