It is difficult to tell from the picture what is going on. The blooms on these hibiscus are short lived and it could be they are simply opening at night and they done blooming.

The problem may be cultural. Have you been fertilizing regularly? Working in some compost around the plant in the spring? I have read that too much phosphorus can cause this condition. You might consider having the soil tested by the soils lab.

http://soiltest.cfans.umn.edu/

If it is a cultural problem, sometimes plants will sluff buds and blossoms simply because they  don't have the energy to support them. For instance, tomatoes do this.

From the picture it appears you have landscape rock around the plant and it is tucked between a building and a side walk. The roots are very limited where they go and support the plant with nutrients. I suggest you move the landscape rock away from the plant, work in some compost and then mulch the plant. Does the landscape rock have plastic underneath it? If so remove it from around the plant. Landscape rock on plastic essentially kills the soil beneath it because the soil receives no moisture and the heat of the rocks cooks the soil.

The problem may be a pest. I suggest you look at the flowers very closely for anything lurking in the blossom. Also look at the stems for any spots or damage. If you do find something please post again.