Which OS, and did it used to work with videos from the same tablet and phone or has it never worked with them, and what do you do in the editor?
If it used to work on the same files then something probably got corrupted and you'll need to fix or reinstall (not sure if that's doable - looks like MM is no longer available). If it never worked with these files then they may be in a newer codec than you have installed for MM (there isn't a single "stock standard mp4 format" - it's really a collection of standards)
For stress on the simple alternatives:
Windows 10's Photos app has simple movie editing built in. It doesn't do much, but it's very simple if you're just trimming and adding titles or graphics.
If your tablet is an iPad then it should have iMovie. Again: very simple if you don't need to do anything fancy.
If you don't mind spending a bit of money then Adobe Premier Elements is good and fairly simple, and there are lots of tutorials around for it. This may be overkill for your needs.
OpenShot is free and has a pretty good reputation. Reviews are that it's easy to get started with, but I haven't used it.
[FONT=Verdana,Arial,Tahoma,Calibri,Geneva,sans-serif]https://alternativeto.net/software/windows-movie-maker/?license=free&platform=windows[/FONT]