Well if you fat travel again and she doesn't show up then she probably died without your knowledge (I found my markarth house carl dead 2 weeks later after he try to walk to my house from drelas cottage). You will have to retrace your steps to the last spot you had her and wait a month.

Thanks all for help. I'll have to try find were I last was with her. Iv fast timed days n days more than 30 but no luck at the black pond. Just have to try n find were I last was thanks agen guys all the best.

I thought Shadowmere only went unconscious and didn't ever actually die?

I lost Shadowmere because I mounted a different horse; didn't know that meant the new horse would replace Shadowmere. I retraced my steps and found Shadowmere at the stable outside Solitude.

Keep in mind that when you fast travel with a horse to a city like Solitude or Whiterun, the horse and you spawn at two different points -- you, inside the city gates, and the horse at the stables outside the gates. Your horse stays at the stables if you switch horses.

Shadowmere can't die UNLESS you drop her off a cliff. A bit of oversight in how she takes damage. She's supposed to be unkillable. She will respawn after 30 in game days though.