What OS and version, are you running PrE 12 on?
Where have you set the Scratch Disks to, and especially the Render Files? Is that perhaps a networked, or external HDD?
Telling us the full details of your computer, and especially the I/O sub-system (the HDD's, in full detail), could be very useful to troubleshooting.
Good luck, and please let us know a bit more.
I can confirm what you are seeing in this specific example when using your Premiere Elements 12. In my case, Premiere Elements 12 is on Windows 7 Professional SP1 64 bit. There is no fix. If you need to get the best possible preview on this footage that was last seen as rendered when you saved/closed the project and that is now presenting as unrendered in the reopened project:
a. render the whole again
b. render a selected segment of the Timeline, setting the gray tabs of the WorkArea Bar area to span just the area to be rendered
c. move on since you previewed this in the previous session and found the preview OK.
In many instance "c" is the suggestion.
The frequency of this type of report peaked around Premiere Elements 7 and 8.0/8.0.1 and seems to have decreased as the versions upgraded to 11. It is interesting to see such a report popping up so soon into Premiere Elements 12.
As in the past, nothing has happened to the preview files that were generated during the rendering before the save/close. They are alive and well in the Adobe Premiere Elements Preview Files Folder when you reopen the project. So, the issue must be in the link between those preview files and the project. The whys were never defined. And, the problem occurence varied for a given project to varying in the details of the Premiere Elements version used and the computer operating system on which it was running.
I will be reporting on all this soon.
Thanks for reporting this.
Just some follow up.
What I have written in my reply 2 in your thread are the essentials of my observations on this rendered/unrendered Timeline content matter.
If you are interested and have the time, you might want to look at further details on the matter in my blog post Premiere Elements 12 Daily Discoveries, Daily Discovery 0009.
As stated, more work needs to be done on the specifics of the Premiere Elements 12 matter. It has the ring of what has gone on in the past. But, best not to take anything for granted.
I will update the blog post with additional significant information on this matter as my search for the how, why, and when continues.