Answer: Pampas grass or Cortaderia selloana is only hardy into zone 7, and according to your zip code you are gardening in zone 6. So it is possible they did not survive the winter due to the cold weather and they would not be expected to have survived.
If you planted the so-called hardy pampas grass, Erianthus ravennae (Saccharum ravennae) which is consdidered hardy into zone 6, I would have expected it to come back into growth about when the tulips bloomed. If you did not cut them back either last fall or early this spring, you should do that to allow sun to reach into the clump and warm the soil to encourage new growth. Otherwise, they may be dead. I would suggest you work with your retailer to troubleshoot the problem.
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