Sorry to hear about the drain. I would be very careful now and protect your remaining drain with as much care as possible, as there may be a higher risk of seroma on the side where the drain fell out.
It is important to keep this drain in place to minimize your risk of seroma (fluid collection), which may then require aspirations and potentially more surgery. I find that drains are prone to issues when a patient attempts to get out of bed/chair or back in during the first few days postoperatively. Try to check your drain to make sure it is pinned or secured to your binder at all times, especially before changing position.