If you're a Google Drive user who has noticed that you don't seem to have all of the files you might have expected, you aren't alone. In fact, a number of people have posted in the Google support forums that they have experienced the same loss of files, and it seems Google is aware that there is an issue.
According to a report by The Register some people have claimed that they are missing as much as six months of business data, something that is far from ideal to say the least. There isn't an awful lot of information on what's going on here, but some users say that the file synchronization from the Google Drive desktop app simply stopped working so the data stored in Google Drive's cloud essentially got cut off at that same date.
A comment by someone claiming to be from Google support told people not to go fiddling with their data to try and get things back after it was suggested that changes to cached files could be what was needed. Specifically, Google says that users should not make changes to the root/data folder to avoid making matters worse.
So far there isn't an official fix here but this is another good example of why storing important files in the cloud isn't always the best course of action. Easy syncing and access can be useful of course, but it's always worth making sure that you have an offline copy of important files as well just in case something strange like this happens to you.