VPJ is not an export format and cannot be played. When a project is saved, that's the control file. It contains pointers to the files and instructions on effects/transitions, etc.
If you saved the project and did not delete or move the source files, just double-click the VPJ to load it into Videopad. Or open VP, click FILE | RECENT in the menu bar and click on your project. What error did you encounter, specifically? If the Mp4 finished export is good in a media player, then it can be loaded into Videopad.
YouTube direct uploads from VP can occasionally fail. In that case simply export in a YT acceptable format (Mp4 should be fine) and use their uploader.
If issues persist, load the original project (not the export) fully onto the timeline. Then FILE | SAVE PORTABLE PROJECT AS, to share with the forum or privately. Someone will help.
Here's how... http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/
If it contains sensitive material link it in a personal message (PM). Click the envelope above. It won't be passed on or retained. Use FILE|SAVE PORTABLE PROJECT AS, not simply SAVE PROJECT, and don't forget to get a shareable link to post.