Can frozen buds kill plants?

[jenlovessun]

New to the forum - really need some advice!

I have 5 three-year old common lilac bushes (three purple, two white) - all are about 2 feet high. Last year and the year before, they all produced green leaves, but no blooms, and all had plenty of green buds ready to go for this year. About 6 weeks ago we had a hard freeze (about 25 degrees). I covered two of the five with sheets because they were the only ones that had leaves beginning to sprout from the buds. After taking the sheets off of them, I saw that most of the terminal growth had been broken off, presumably by the sheet I used to protect them. Now 3 of the 5, including the 2 I covered, have bare branches (the leaf sprouts have shriveled and dropped) and brown, dry buds. Should I trim off the tops of the branches in hopes of better results next year, and if so, how short and when? Should I not have covered them? Can this lack of leaves for the season kill the plants? What can I do to save them? Any advice would be appreciated!