Completely logged out of everything on my phone???

Not sure what happened today but I woke and my phone was working perfectly fine. Brushed my teeth, came back and then all of a sudden it was asking for me to re-input my email password for the mail app. I thought it was weird and then it asked me to do it for all my emails. Strange!

Then asked me to sign into iCloud. I Noticed I wasn't connected to my home wifi anymore so had to reconnect to that too.

Got in my car and had to pair my phone with the car Bluetooth again as it has lost that too. Same with my Bluetooth headphones.

Got to work and had to re-sign into the work wifi network.

Basically every single app on my phone has signed out.

Even my internet banking which had my device registered for ages and no problems, I then had to re-register my device with that too. It's as if every single thing on my phone was new but I have been using them fine for years?

This is a massive ballache having to sign into everything and trying to remember passwords and register devices again etc etc.

What the **** could have caused this and how do I stop it from happening again??