A sudden dropping of leaves usually results from some environmental factor— herbicide drift onto leaves or something applied to the root zone.
https://extension.umd.edu/resource/herbicide-damage-trees-and-shrubs/

If you think this was the case, follow the instructions in the bulletin, above. It may take until next year for the tree to recover so, keep the tree watered during any times of drought( more than 10-14 days without rain.)

If you want us to take a look, please attach a picture of the whole tree taken far enough back to see it from the soil line to the top of the crown.
Then, please take a close up of several,leaves, some face up some face down, so we can see the damage. 

To add pictures, click the link in your email and go into your question on the website.

Click the “Drop files here or choose them” below the response box to attach them.

You can upload .jpg, .png, .gif, .txt, or .pdf.

NOTE- each picture must be 8mb or less.

Very detailed and helpful. I'll be back at you...for now my neighbor is coming to my rescue with a saw.