Q. Why are the leaves on my hibiscus turning yellow?
You don't indicate whether your Hibiscus is tropical or hardy and whether it is in a pot or planted in the ground. Also, are all the leaves turning yellow?
If it is tropical and in a pot, chances are that it is from overwaterring or over fertilizing. If it is outside, chemical treatment of the lawn may have drifted onto the plant, causing the yellowing of the leaves.
At this point it is probably best to monitor the plant to see if the new growth is normal. Do not water excessively or apply fertilizer.