Any advice on how I can save this Clematis that has blown down. Should I cut it back and build a new arch?? If so how much should I cut it back.

I have cut clematis to ground level and they come back with vigor.

I won't cut it to the soil level, count up from the bottom at least 3 leaf nodes. Around
6 to 8 inches. Then rebuild your new arch, so there is better support next year. I would also remove the grass around the base and mulch to keep the grass out. Just my 2 cents worth.