Note that the first thing you should do if a horse starts to lift both front feet is lean forward, drop the reins, and apply both legs firmly to send the horse forward. Another thing that can work if you have the balance is to free a hand and gently press on the top of the horse's head...I stress gently. Never hit a horse on the poll - it's so vulnerable you can actually kill them.
And then once the horse is behaving, work out why it started rearing. The most common reason for an otherwise well behaved horse to suddenly rear is pain - usually in the mouth or the withers. Check saddle fit and teeth.
