15 month old crying for HOURS at night

He used to be a pretty good sleeper or he would cry when we would put him in his crib (in his own room) for like 20 mins and then knock out. He will be exhausted, and this little guy will cry for HOURS!! 3 hours straight last night! Would go in every 30 minds to rock him and put him back, only for him to be even more pissed and scream louder. No fever, gave him meds for teething in case that’s it , has his water cup, is fed, changed. He is relentless! We are sooo confused. He’s been like this for a week-ish. Good lord! does anybody have any suggestions?