Call your local extension. They'll have more info about your specific area and whether there may have been a chance of frost damage, currently active pathogens or blooms, or critters active in the area that put your plant at risk.
A few other things:
- Get the soil tested. Take 3 samples from around the plant.
- Check the moisture 6" down. If it's too wet, check and see if you have spouts or water leaks in that area.
- Are any other plants in the surrounding area struggling?
- Get some basic slow release organic (mostly manure) fertilizer and spread it around the canopy range of the range according to directions. If it has an infection of some sort, this will at least help it defend itself.