I am using ColorMag WordPress Theme. I want to add a new post. I click on add new and then I type the title and then write some content for that post. then I click on Edit With Elementor button. But when Elementor interface open all my content has vanished and there is no post title showing on that.
This is also happening if I try to edit my old posts with Elementor.
It sounds like your theme is automatically making adjustments on the posts where you are using Elementor. Typically you would want a blank canvas, just the header/footer, when using Elementor. So this is most likely normal behavior.
Many themes have an option box, where you can set up your post or page. I have not tested the ColorMag theme yet.
It is normal though that when you have content in a post or page, then want to use Elementor, that this content is not brought in.