Three things to try, in turn...
First Save Project File AS. Do this routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, that choice is available by clicking the caret to the right of the Save Project button on the toolbar.
1. Close Videopad. Relaunch, but don't load anything. Open OPTIONS | DISK tab and Clear Unused Cache Files.
2. Reinstall the program.
3. Install the latest release. For licensed users upgrades are free for up to six months from purchase date. After that, VP will continue to fully function but a fee will be required in order to register the newest. Retain your old install file and registration info.
If none of these measures is the fix, come back here.