TV signal lost - new aerial plate fitted

Who installed it? Maybe they're liable to fix it?

It could just be wired up badly and the cable/braid could be making contact with the plate. If you want to check yourself, unscrew the wall plate and pull it away from the back box, then check to see if the aerial cable coming in is secured into the relevant terminals on the plate. (edited)

Did you replace the shorter cable that goes from the wall plate to your TV? It might not be making good contact with the new plate.

When we redid our aerial a month or so ago I ended up replacing pretty much everything as I worked down the line, as I disturbed each connector I found connecting new to old just didn't work.

Start by double checking the connection to your TV & then after that socket wiring & then go into your loft to see if you have dislodged aerial connectors.

As you have lost pretty much all stations it is a connection issue.

I bet you haven't connected it to your tv as before. (edited)