Thanks for writing. Without being able to see your plants, I can't say why they didn't make it. Do you know what type of hibiscus they were? The most common hibiscus tree is a tropical plant that's not hardy in your area. The type of hibiscus tree you should be able to grow in your area is rose of Sharon (Hibiscus syriacus).
Some reasons that your plants may not have come back if they were rose of Sharon include: If your winter was especially severe; if you planted your trees too late in fall and they never had a chance to get established; if you had an especially cold winter (Zone 5 is just an average temperature rating. A colder-than-average winter would make your yard a Zone 4 one season....); a pest such as voles attacking and eating the roots.
---Justin, Senior Garden Editor, BHG.com