There are quite a few possibilities but there is one simple thing you could check first; Is there any chance that you got some snow or ice buildup on any of the wheels since the last time you drove it? Snow or ice frozen inside the wheel or hub cap causes an imbalance and can result in a mild to severe vibration at higher speeds.
If you rule that out, it might be wise to have it checked out by a qualified mechanic. It could be as simple as a thrown wheel balance weight or something more severe like tread separation in a tire. Certain suspension and bearing issues could also be at fault. Those are best left to a qualified mechanic to address.