Go back to your original project and your original video will still be there... as long as you did not delete project or original files
All is not necessary lost. The answers will be in the details.
What version of Premiere Elements are you using and on what computer operating system is it running?
What are the properties of your source video - video compression, audio compression, frame size, frame rate, interlaced or progressive, pixel aspect ratio? The quickest way to get this information is to know the brand/model/settings of the camera that recorded the video. If you do not know that then we will ask you to download the video audio properties readout program MediaInfo (Tree View) to obtain the information.
What have you or the project set as the project preset (which should match the properties of the source video)?
What is the total duration of the Timeline for this problem project? Are the originals for the files imported into this project still at the hard drive save location that they were when they were imported into the project?
What is your intended export - burn to disc or export to file saved to the computer hard drive?
What were your export setting for the mpg export that looked so bad?
What you have imported into the project are just copies of the originals on the hard drive. And, the project needs to
keep the trace back to the originals; otherwise, reconnect problems set in which prevent you from moving forward
in the project or even opening the project.
Please review the above, answer the questions, and then we can plan troubelshooting strategy.
If you need clarification on anything written, please do not hesitate to let us know.
Just an add on to my post numbered 2.
I think that John has misunderstood your question. So, please confirm
I am interpreting your last question as "How can I get an export with the same quality as the video that I have imported and edited
in the project?" And, not "How do I find on the computer hard drive files that were imported into the project?"
Whether or not it is going to be worthwhile to "save" your edited Timeline and do another export with better export settings is a question which will be answered by the details already requested.
Looking forward to further details and clarification.