Question

"Death lies on her like an untimely frost upon the sweetest flower of all the field." (Romeo and Juliet act 4.5)

Answer 1

It seems like the character feels Juliet was too young and beautiful (like a flower) to die. With her untimely death like an early frost.

Answer 2

This was obviously a long time ago, given the English. The character seems to be upset and he finds this loss to be a great one. He clearly loves this girl and he thinks losing her will be a tragedy. He is calling the girl the 'sweetest flower' and the 'untimely frost' is her death that happened too soon, with her being so young.