OK, several things going on here.
Firstly, it has fallen over. This is not a problem. The original Bramley (sown 1809) fell over in 1900 and re-grew from a branch, and is still going strong.
Looking at the tree, the photos don't make it entirely clear, but it looks like there is not much in the way of annual extension growth and the centre is getting very bare, which means the branches are probably getting over-extended (which is consistent with your comment on broken branches).
The first thing I would do is cut everything down around the root zone. This will also make working on it easier.
I wouldn't touch it until late autumn/early winter, when the leaves have yellowed and mostly fallen at the earliest. There is a particular range of airborne spores around at this season which you want to avoid landing on freshly cut surfaces.
I would then remove any dead wood, and shorten up anything particularly long. I can't identify any particular cut points from the photos, but in your first picture you can see a point about two thirds of the way up where the branches arch right to left and another set carries on up. I would look at taking out the ones which are heading on up. The ones which arch also look quite dense, so I would look to thin them a little - helps air get through reducing wind resistance and lets light get to the branches so they don't die off. This will all help to reduce leverage on it, making it less likely to blow right out of the ground through twisting, or for new bits to snap off. Coincidentally it will make it easier to pick and manage in future.
In your last picture, there is a horizontal branch running right to left, which then turns upwards. At the point where it turns upwards there is a young shoot. This is the kind of thing you want to encourage. For now, you are looking to get a few of these going, which will happen if you head the tops back and let light in. In subsequent years you will be able to thin these out to space them to form new side branches. You want to encourage them to grow at about 45degrees to the vertical, so choose the less vertical ones to leave. If you have to leave some vertical ones because they are well placed, bend them down to the right angle and tie them there with a bit of string (in the growing season they will 'set' to their new angle in about 6wks). This will ultimately allow you to cut right back to your new branch structure, forming a more compact tree which is more stable.
To get it growing again, in about March I would stick down a double thickness layer of cardboard all around the root zone, which might be the drip line but having fallen over you will have to guess at this a bit. This will stop it competing with the weeds for water and nutrients. Before I did so I would give it a decent dose of Blood Fish and Bone (or Growmore) so it's a good slow-release balanced fertilizer. I would then mulch over the cardboard with either well rotted compost or chip/bark which had been stacked for a year - if you don't then the cardboard dries out so the rain doesn't soak in as well, and it blows around. Cardboard lets the water through but suppresses the weeds. The worms like the glue so they pull it down, aerating the soil. This means you will have to renew the cardboard from time to time, just don't let the mulch get too thick.
The above is likely to give you a lot more years out of it.
Alec