If the back pain is not caused by, e.g., skeletal problems and is instead caused by simple compression, riding a drop-bar bike is one of the few ways you can exercise while reducing disc pressure, so it's worth figuring out how to be comfortable on the bike.
Maybe you rode too long for your first attempt after a long layoff, with your back reacting to holding your upper body in one posture for too long. You might try doing just 10 minutes or so per session until you get re-accustomed to riding in that position.
Maybe you were pushing too hard. Some people mistakenly think that indoor riding should be performed at high resistance levels, as if riding a stationary bike should feel like working out on a leg press machine. Current thinking is that most indoor riding should be performed as aerobic exercise and done at a moderately low intensity and moderately high pedaling cadence.
And maybe your position on the bike isn't quite right. The most common error is positioning the saddle too high. If your hips are rocking as you pedal, try dropping the saddle until they don't. You can experiment with your reach to the handlebar, handlebar height, etc., but start with checking the saddle.