20 month who used to sleep

Having trouble lately, my son used to be a champion sleeper but lately he wakes up around midnight every night and screams. We tried to let himself soothe but he’ll scream for 2 hours if we let him and, we did once or twice. He’s not hungry or needing to be changed. He naps once a day for a decent amount and has long wake windows for his age. What else should we try?