What computer operating system are you using for your Premiere Elements 12? Did you ever update it to 12.1 using an opened project's Help Menu/Updates?
Please review the following Premiere Elements 12 will not open solutions and determine if any of them work for you.
Please give more details, including....
1. Does the problem apply to old and new projects?
2. Does the problem exist with and without antivirus and firewalls disabled?
3. Do you have the latest version of QuickTime installed on your computer along with Premiere Elements? And, are you running the programs as administrator (from user account with administrative privileges)?
4. Is your video card/graphics card driver version up to date according to the web site of the manufacturer of the card?
Let us start here, and then decide what next based on your details and results. Any questions or need clarification on the above, please do not hesitate to ask.