The answers are in the details which are lacking in your report.
What version of Premiere Elements are you using and on what computer operating system is it running?
Please check out the Adobe troubleshooting documents for this issue
After you review those documents and determine if either suggested troubleshooting scheme will work for you,
then we can discuss the immediate and future actions.
Looking forward to your follow up. Depending on your reply, next posts may require of you details
a. project settings
b. properties of your source media
c. edits and inspection of Timeline content with the Timeline expanded with the -+ slider above the Timeline.