Answer :
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.
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.