If mp4 export isn't available in the 'free' version, AVI usually succeeds. Reloading the program is often all that's needed.
You may be trying to play the VPJ file that saves instructions on where to locate the source files and how to handle transitions and effects. That file is not playable.
Nothing will be lost if the VPJ was saved (the program prompts for that if editing has been done). But it's always wise to Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite.
With Videopad installed, double-clicking the VPJ should open your project in Videopad for further editing or exporting as long as you have not moved or deleted the source files. Or you can launch Videopad, then click Menu (upper-left), FILE | OPEN PROJECT and locate your VPJ project file.
Normally the VPJ file is in the same folder as the source files, but you may have to search for it. If necessary, search for "autosave.vpj." That's the protection file that Videopad saves in case of a crash.
Drop in again here if these measures do not succeed.