Mistakes or technical problems with media management are almost inevitable even for the most well organized editing project. The fact that media is separate from the project file in all editing programs leaves space for source files to get deleted by accident. If you happen to delete the source video for some of your captured footage there is a simple process you can follow to repair it.
A deleted source file can be a major problem, especially if the video is already cut up and dotted throughout the timeline. From here you are going to need to take the source tapes and recapture them. The best way to do this is to open a new project in your editing program and begin to capture the tapes that were lost. Name them something comparable, but indicate that they are recaptures. Make sure to then find the base file of what you have just captured, which will be in the capture scratch folder of your project scratch folder. Go back to your other project and go to the disconnected files. Select them and then reconnect them to their comparable footage that you have just recaptured. As long as you have not changed the time code settings so that time code goes to zero on each capture, the time code should be the same for the video you captured before and the new base video that you have in the capture scratch. Now the video in your project will reference the new video you just captured, which is the same and shares the same time code.
The process will be somewhat different if you were doing a Log and Transfer from a digital storage card, like a CF card. When you transfer in these clisp they will come in as independent clips on their own, and as long as you do not interfere with the capture process you can simply recapture them into your project and they will be exactly the same as the previous capture. You can then tie them to the clips in the timeline, though you will have to re-attach them to the capture scratch. You will luck out again because all the clips will have the same names they had before because this was determined in the storage card itself.
This can be difficult if the master clips in your browser are smaller than the entire source video. This should not be a problem as long as the time code does not restart on each capture attempt. Reconnect the media on each master clip and then each smaller clip, sub clip, or video cut up in the timeline will also reconnect immediately. This can be quite a bit of work, but is a lot less work than going through all of your footage, reimporting, and then cutting up the clips again. Since your creative work in your editing program is simply a bunch of marker points you have to make sure that all of your raw materials stay in tact.
The most important thing to consider is that the footage that is disconnected may or may not actually be deleted. The process for most nonlinear video editing programs, such as Final Cut Pro, is to link to existing full video files in the program. The program then essentially plays a preview referencing the actual video file. To do this the program has to know where it is, and if it suddenly cannot find it then it will not be able to connect. This will give you the famous disconnected video image, though you may be able to reconnect by selecting it and finding the file. Before going through a recapture go ahead and walk backwards through your project. Did you set your capture scratch to an internal hard drive rather than the portable hard drive you are working off of? Did you suddenly move a folder in your project file? Did you rename a series of files? All of these could be responsible for this, and the footage could still be there.