Zone 5 last frost date is generally April 15, but you need to remember this is highly inaccurate.
Weather trends and observations are very important to carefully watch early in spring. Evening temperatures need to be around 50 to 55 degrees for your plants to grow well.
Did you experience below freezing temperatures or frost dangers after planting out?
If your tender young plants were exposed, most likely there condition is due to freeze.
Your season is very early yet and I would not hesitate to start again.
Here are some links to read through.