Ice (but not in contact with the skin) is best during the first 72 hours) then heat/cold whichever feels more comfortable. Avoid long periods of inactivity, just keep moving/resting.
To get some mobility back, do some easy stretches (loads on the Internet).
As your mobility improves, do some core excercise again there are loads on the net.
Long term, try to concentrate on alternating your leading leg/foot. Most cycling back pains are caused by muscle imbalance and most people always lead with the same foot on non-pedally descents. This results in one leg that is made of steel and similarly inflexible while the other leg is just there for the ride. Hello back pain.