All the cold taps upstairs and the toilet are now running hot. Cold water tank in the loft is full of hot water!

Help!

Any changes prior to this happening, especially to the hot water heater?

jlisenbe asked,"Any changes prior to this happening, especially to the hot water heater?"
I would ask the same thing about ANY mixing faucets that have been replaced or repaired. This sounds like a cross connection. Regards, Tom

No changes. Just happened yesterday. I have switched off the emersion heater and run the taps to empty the hot (steaming!) water from the cold water tank in the loft. Now the water is cold. Tried turning the emersion back on and it went pop - like a lightbulb sound. So switched it off and awaiting plumber.