Lots of help.
If a purchased version of Videopad is uninstalled, it can be reinstalled and reregistered if necessary. No need to pay again. Upgrades are free for six-months from date of licensing. https://www.nch.com.au/support/reg.html
As to your issue, download and install this version. That should solve the missing video problem.
Save your project first, of course, using Save Project File AS, and giving it a unique # or name, so as not to overwrite prior saves. Clicking on a VPJ file will load the project as of the date the file was written.