If you move files or folders around your Google Drive in the web interface, you may lose them. They just disappear mysteriously.
According to a discussion thread on Google Group, this issue has been arround for almost 2 years. And there are some ways may help you to get those files/folders back:
0. Wait for 24h and your files/folders may be back (from nowhere).
1. Search in the Trash.
2. Search with "<name of the files or folder>*" keywords.