It is possible to overdry. Turning brown is one indication. You're really overdry if the bracts (the "leaves") start falling off the stig (the stem). When this happens, most of your lupulin (the yellow/gold stuff) falls out. The lupulin is where the good stuff is. sometimes you can also get to the point where the cone opens up too much and the lupulin falls out even though the bracts are loosely attached.
If your hops are still intact, you should be OK especially since they still smell good. If you're worried, you can condition them just like the big boys do in Yakima. They just lay them out and cover them with a tarp to let the moisture equalize. But since you may not have enough moisture, leave them on the drying trays. Fill a dish with water and some water conditioning salt. Set the trays on top of this and throw a tarp over it. Leave this for a day or so. The salt should equalize the relative humidity to around 60 to 70% which will give your hops an internal moisture content around 12 to 15%, which is perfect.