Are they on an external drive?
If so, you will need to relink them if the external drive doesn't have a specific drive letter assigned.
Have you moved or deleted or put in folders the source media after you saved closed these projects?
Can you still see the project's source media on the computer hard drive in their save location?
If so, what is that saved location...external hard drive or somewhere on the Local Disc C?
As suggested in Peru Bob's question, if external hard drive is the storage place for all your source media, has the drive letter changed since you saved/closed your projects?
By the way, what computer operating system is your Premiere Elements 12 running on?
Looking forward to your follow up.
Welcome to the forum.
For Media Off-line, see this Tips & Tricks Article, for more background, and suggestions: http://forums.adobe.com/message/3561281#3561281
There are also links to other articles, such as the "Where is file ___ ?" error message, that might be useful.