How do I fix an iPhone 6s plus that has lost apps?
A normal shutdown is in order. Press the sleep wake and slide to power off. 

If that doesn’t help and you have an iCloud backup and can erase all content and settings, I would try that. If not, restore when you connect to iTunes and then restore a backup. 

If you don’t have a backup, you might attempt one via iTunes first and hope you don’t lose all data.