How do I fix an iPhone 6s plus that has lost apps? My iPhone was not working, so I decided to force reset it (pressing the home button in sync with the power button)

After this most of my apps were gone (including the settings, phone, messages, Facetime, etc). 

How do I fix this?
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.