Since the start of air travel quite a number of planes have gone missing, as you will see from this list. The list does not include aircraft that went missing in action, as it would then be a lot longer. However, if you look at the dates you can also see that the closer you get to our own time, the fewer permanent disappearances you find. This is because modern technology makes it much easier to track aircraft and to locate them if they do go missing.
In fact, before the current disappearance of the Malaysian passenger plane in 2014, there had been only two unexplained disappearances in the 21st century. These were a stolen plane in 2003, last seen near Angola, and in 2008 and aerial survey plane that disappeared in the vicinity of Georgetown, Guyana. No good explanation has yet been found for what happened to either of these two planes; it is too early yet to say whether this will also be the case with the current missing one.