Help needed for my oak tree

I had a healthy oak tree in my backyard for about 6 years now and the lawn mower pruned it two weeks back and we had gone on vacation.
When we come back, we see that the tree has lost all its leaves (in the month of August) and there are tiny drops that looks like sticky gum in the trunk of the tree.
Please can someone help us understand what's going on with the tree ?

Thanks much.

Vani

It sounds like borers have really invaded that tree.  
I'm not sure I understand the "lawn mower pruned it two weeks back" This is not the time of year to be pruning trees.

Thank you for responding. I agree the mowers shouldnt have touched it. I am attaching a picture, please can you confirm if this is something hat can be treated? It has something, like a white froth.

Thank you for your precious time and advise.