You've moved the clips from the folders they were in when you edited them. You've got to point Premiere in the right direction to find the files. When you load the project, it will ask you where the files are that it has offlined. You select them in the dialoge box and it will find them.
As stated, Moving/Deleting are the most common causes for Media Offline. Now, if you have the media files on an external HDD, and turn it off on occasion, while adding another, or some device seen with a drive letter, your OS will assign a different drive letter. The drive letter assignation can be done in the OS with Computer Manaement (Local) and Disk Management. This will insure that that external always shows the same drive letter. If you use this external on more than one computer, you will want to do this in each computer's OS.