Answer: There are several different kinds of hibiscus. The perennial hibiscus flowers die back to the ground every winter and then re-emerge in late spring when the ground has warmed up. They are usually one of the last things to come back after the winter. If this is what you have it is probably still a bit early to worry.
If you have the hardy shrub hibiscus Hibiscus syriacus, it too, is late to come back into growth in the spring. Sometimes it will suffer dieback during the winter, especially if there are oscillating temperatures. You can check on it and remove any winter killed branches. Start at the top and work your way down the shrub, removing dead wood. Live wood is supple and has green inside the bark. Dead wood has off color brown or gray bark and is brittle, it will snap in your hand. Prune away any dead wood and then if necessary prune a bit more to get a somewhat symmetrical look. This shrub will bloom for you this summer even if you have to trim it back a lot now.
If your hibiscus are the tropical hibiscus plants (Hibiscus rosa-sinensis) then they would not have survived the winter outdoors. They do not survive freezing temperatures, sadly.
I hope this helps you trouble shoot your hibiscus.
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