There is no button in image (compositing). Button called mist When I rendered animation (450 frames), I would like to composite it and add mist.
I closed Blender and than reopened it again. But when I go to the compositing and add image node with image in it, I didn't see mist button.
What can I do? I rendered animation for 4 days, so re-rendering isn't my option.
Short answer. Re-rendering is your only option.
PNG will not save the mist pass, or any other passes. It will only save color and alpha by default.
If you need to save multiple layers use OpenEXR multilayer.

When you import the rendered image back it will have the corresponding sockets for the layers you saved.