A photo and a good close look at the leaves for any pests will help. Then have there been any adverse weather conditions such as too much or too little rain, excessive heat, etc. This will help members in determining a cause.

Welcome to GoY, Laraine!
A few more details would help, though:
What were the exact symptoms while the leaves were turning brown? How long has the tree been in the ground, or is it in a pot? If in a pot, how often is it watered? Did anything notable happen near the tree recently, such as digging, construction, new pavement or paths, etc.

A couple of years ago, my apple tree leaves all turned brown during July. They all fell off during August. It was caused by Apple Scab. I sprayed it the following two years and have so far not had any recurrance, but I think it will be an ongoing battle. The leaves had dark brown spots on to begin with, but eventually they all died and fell. So Google 'Apple Scab' images and see if it looks the same. Otherwise, the advice above is all grand.