The Hero monument in Mirra w Thur wont activate. can anyone help please

If I remember correctly, that one often has a goblin hiding behind it, preventing activation.
Though the activation button should say whether there are nearby enemies.
Try doing a walk around it, and see if you can provoke any hidden critters.

If that doesn't help, stay near it and press Tab. That will switch between hostile targets inside your current view screen, starting with the highest level enemy and cycling through them, even if you cannot physically see them.

Thanks for the advice but it did not solve my problem. I can detect no nearby enemies and it still won't activate which leaves me stuck in the game.

Just to make sure, which of the Hero monuments are you referring to?
The one to the lower-left on the map, or the one in the middle?

Could you post a screenshot?