What you describe is common to all versions of Premiere Elements. It represents a disconnect between the source media that was imported into the project and the project (project.prel file).
When you import media into a project, you have copies in the program. The copies trace back to the originals in their hard drive save location. Typically the situation that you describe happens when you move or delete the original files from the hard drive after you have saved/closed a project. An alternative explanation might be that you have your source media saved to an external hard drive and that drive's drive letter has changed spontaneously for some reason.
Do the source media for this particular project still exist? Can you find them on the hard drive where you saved them when the project was opened last? If they still exist, you can use the "Where is the file....?" messages that you are probably getting to reconnect the source media with the project.
Please review the above and then, based on your reply, we can decide what next.
Welcome to the forum.
I agree with ATR - "Media Offline" red screen. In addition to his comments, there might be a little more info in this Tips & Tricks article too: http://forums.adobe.com/message/3561281#3561281 If you are using external HDD's, see the link at the end for some tips on using those.
Also see this Tips & Tricks article for "Where is file ____ ?": http://forums.adobe.com/message/4598705#4598705
Lots of information out there to help in understanding the origins of those media offline colorful displays and "Where is the file....?" messages and related matters. Just two besides what has already been posted....
for avoiding situations such as those
as an alternative to the "Where is the file....?" message for the media reconnects
But, first we need to define your particular situation and the background on how you arrived at where you are. So all those specific details that you supply will direct us to what in all the information is applicable to your situation.
Looing forward to your follow up.