Is the pump running? Have you managed to disconnect or wrongly reconnect the pump when you did the electrical part of the valve replacement?
If the pump isn't running, disconnect it from the control box, put a 13A plug on it and then plug it in - if it works, then the connections are wrong somewhere. If it doesn't work, you have another job to do.
If it does work and the radiators don't get hot when the heating is switched on, you have a seriously bad air-lock.
Be careful to make sure the pump is or is not running at each test - it should run very quiet so it isn't always very obvious - the old screwdriver held to the casing and the other end of it to your ear can be helpful.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!