I have a climentus vine that needs trimming. Can I trim, how and when?

Wait until the new growth starts to emerge so you can determine what dead wood needs to be trimmed.

I wouldn't cut it back that hard unless it has frozen to the ground. Look for new buds and cut back from there. Think about how much it grew last year and plan for that. Clematis blooms from new wood, so it needs those little buds to grow.

This article should help you get started-https://www.gardeningknowhow.com/ornamental/vines/clematis/pruning-clematis-vines.htm

You will be able too see the dead part of the vine once it starts emerging leaves.that is all that need to be pruned

If you mean a Clematis vine, you need to let it grow and determine if it's the variety that grows from old wood or or not. Let it grow, if it forms buds on old wood then you can clearly see which old dead wood to cut. If it only gets shoots up from the ground, then you can cut all the old vine.

blossems have fallen off, do i dead head it

https://www.wikihow.com/Prune-Clematis#:~:text=To%20prune%20spring%20blooming%20clematis%2C%20like%20Alpina%2C%20Montana%2C,and%20removing%20the%20top%20part%20of%20the%20plant.

Hello, there are different type of clematis, most of them prefer not to be cut. You should wait until they buds come out in the spring, then only trim the off the vines that do not bud or bloom. Once it has started to grow, use ribbon or something to mark what needs to be cut off in the autumn when it loses its leaves.