What will happen if you bleach your hair and two days after you dye it with ash mahogany dark brown?

Depending on how porous your hair is, it will probably grab this color and be very, very dark and greenish.

If you want to go back to brown, you could use a Level 5NG with a Red filler.

This is difficult to do at home and should really be handled by a professional, but you will need to replace the missing pigment that the bleach removed from your hair. Using a Level 5 Neutral Gold and adding Red, with a 10 volume cream developer, should give you a nice rich light brown.