Home DIY

It's a 3 port valve for a Y-plan system.
The metal box contains a motor which turns a valve inside the pipework joint.

They can fail and you can remove the metal box by loosening a single screw. Be aware it's mains voltages running through that cable so power down the boiler before doing anything with it.
You can also replace just the motor if it's burnt out.
There are quite a few screws but most of them you don't need to touch - the motor is only fixed by one iirc.
Hopefully it doesn't need replacing - they're more expensive than you'd wish.

The metal lever probably won't visibly move. You can move it by hand to the 'open' position and latch it there by hooking it over the little metal tab. There will be some resistance as you need to spin the gearbox. Usually you need to do this if you want to drain the system.

If there's no resistance to moving the lever, the motor is probably activated and the valve is open.

Engaging the hot water heating or the central heating should cause the motor to activate (you'll likely hear a relay click in the heating controller, the motor whirr, then a click as an arm makes contact with a microswitch to tell the system the valve is now open and it can turn on the pump and fire up the boiler.