That does not sound good. Did you mean that the plant leaf out from winter ok, the leaves were green and now they turned brown? Did you notice how the plant leaves transitioned from nice green to all brown? Can you post a picture of the hydrangea bush and identify the tree? Also check "under" the soil for insects, grubs, etc. but be careful not to disturb the roots too much. Check for mites in the leaves or insects that cause the leaves to curl.

Is the soil pH within an 'ok' range? Is there anything unusual about this planting area from an old construction standpoint? Something that could have injured the roots? Have you looked at the stems from top to bottom looking for injuries in the bark?

There could winter issues that are now surfacing too, watering issues (but the signs would not be limited to the bottom of the shrub) that cause root rot, etc There could also be issues with tree root competition or problems causes by trees that release juglone. Compacted soil/rocks may hamper the small feeder roots from doing their duty and cause areas of the shrub to brown out too.