Patricia - From the pictures, it appears that the leaves that are forming do not show signs of disease.  See the article below for typical diseases that can affect Hydrangeas.

https://extension.psu.edu/hydrangea-diseases

Possibly this is die back from a cold snap in winter.  That would mean the upper part of the plant was affected but not the roots. Hydrangeas can grow in morning sun and since they have been doing well for the last several years, I would not focus on that.

Whether the upper stems are live or dead, you could encourage new growth by pruning each stem back to an active bud node (where a leaf is coming out).  This may mean cutting to the bottom of the plant but if the shrub is still viable, you should see new growth within a few weeks.  However, if these Hydrangeas bloom on old wood, you will sacrifice the bloom for this year.  Provide some Nitrogen fertilizer and make sure to keep them well watered until the growth appears normal.   It looks like you already have mulch around the base so make sure to keep that so the soil stays moist.

Hope that helps!