There may be easier ways to fix this but this is how I’ve done it in the past:
Open Music and go to Songs view. Right Click on the first song choose Get Info and review each item in the Detail and Sorting tabs. Continue through the entire list. Something will be different on one or more of the fields in each of the “split” songs. That is what has divided the CD.
Once all fields in all songs are the same the CD should go back together. Copy & Past will be your friend in accomplishing this.