Hi, you don't say which species of Clematis it is, there are around 200 species, divided into 3 groups, each 1 of which has a different pruning regime, group 1 Clematis flower on the previous years growth, and flower in spring, these should be pruned after flowering, removing dead or damaged stems, and cutting back all other shoots to where you want them.
Group 2 Clematis flower on side shoots from the previous years growth in late spring early summer, and in mid and late summer at the tips of the current years growth, these should be pruned before growth begins in early spring, again cutting out dead or damaged shoots, and trimming all the other growth back to where you see strong buds.
Group 3 Clematis which flower on the current seasons growth in summer and early autumn, should have all the previous years growth cut back to a pair of strong buds 6-8" from the ground, before growth begins in early spring, hope this helps, Derek.

Thank you for all that information but I am not sure which species it is. I have only been here since April 5th 2014 and do remember it flowering about end of April early June. But it was necessary to cut a great deal down from the wall as it had grown under the eves and damaging the roof. It is obviously very old and has been neglected in recent years.A winter storm ( I'm 1000 ft up) pulled it away from the wall and as you see and it blocks the way. I really need to do something about it now or soon. I don't want to kill it but nervous about cutting it back incorrectly.

If it flowered around April, and the flowers were relatively small but numerous, its likely a montana variety - google Clematis montana and check the pictures out. This one can be 'renovated' by cutting right down after it flowers, or just pruned normally, but it should really be done after flowering.

If its really in the way, then you might just have to take a chance and cut it right back now - you'll get no flowers this year, and it is still a bit early to do it from the temperature point of view. If you can bear to wait till mid March, there's more chance of the plant surviving and regrowing. Once you've cut it back, then is the time to provide a proper support for the plant - I can't see any evidence of any trellis or anything currently. Also bear in mind that if it is a montana variety, it will want to get up to 35 feet high.

That looks like one heck of a main trunk. Given the complete tangle it might be worth cutting it all back to a couple of strong stems with good buds and (probably) forego flowers this year whole it re-establishes itself. Fix a good trellis or some wires for it to cling to as it re-grows.

Is that a drain cover next to it?