Actually I, and I am sure several others here have a good idea what the problem might be.
But, you pratt, your tenant has already reset the POS boiler as you have disclosed in your opening post.
Now sort your tenant out and pay a professional in order that you uphold your contract with your paying tenant and give the poor sod some heating and hot water back.
Now run along, there's a good boy.
How do you know what contract I have drawn up with my tenant? For all you know I might not even be responsible.