When say iPlayer doesn't load I assume it's not just asking you to sign in again.
I guess you have already tried rebooting the box by turning the power off and on again.
You could try a factory reset,
It's only a partial system reset and not a complete factory reset. You are given 2 options,
Choosing Keep Recordings, means you won't lose your recordings and any planned recordings or series links you've set up, but you will have to sign into your apps such as iPlayer, Prime, Netflix and ITVX again, also Match frame rate and Standby power consumption will go back to their default settings if you have changed them.