Nothing good. Either they try to escape and are caught and killed or they escape and are stranded.
If they are caught while trying to run, they are killed on sight. No trial, no muss, no fuss. Just death. They wouldn’t be worth the effort if they were to try to escape again. It is acceptable to simply kill the humans for disobeying the monster, so of course they would die immediately.
If they are able to escape their kidnappers, which is extremely, extremely rare, it depends on where they are located to see if they will survive or not and how many resources they have available to them.
If they are behind the barrier, it is impossible for the human to cross without a monster physically holding onto some body part of theirs long enough to cross through the thick layer of magic. It is illegal for a monster to cross the barrier with a human without a permit issued by the King.
The reason this works, why a human must grab a monster to leave, is because shortly before they were brought over to Nocturna, a spell is secretly cast on them to prevent them from exiting the barrier by themselves. It acts as a barrier in itself, trapping them inside Nocturna once they cross over it for the first time. Grabbing onto a monster confuses the magic because it’s getting the non-magic signature of the human but also the magical signature of the monster, so it doesn’t activate.
Humans have no knowledge of the spell. It can be removed by spellcasters, but that is also illegal. For those who know what to look for, it looks like a band of clear, flowing energy encircling the human’s wrist. There is also an alarm when the bracelet comes off, almost like a dog whistle, but monsters can hear it instead.
The human trying to escape would be hard pressed to find a monster willing to cross them over, so it doesn’t usually work out. They are killed if they are stupid enough to grab a monster the second before they cross the barrier. The humans can also attempt to cross alone, learn that they cannot cross the barrier, and are forced to wait until traders find them and are killed, or they starve to death because they can’t return to where they ran from.
If they are not behind the barrier, the humans wouldn’t be able to go home as the memories of them would be forgotten due to the potion they were forced to take when they were kidnapped. They could try to convince everyone that monsters exist and be thrown in an insane asylum, where a monster would come and dispose of them.
Their last option would be to start a new life from scratch, create a false identity, and live the rest of their life peacefully (this is a rarely taken option because none of the humans either know about what happened to them or they just didn’t believe it).
Any way they choose, they will eventually be killed. It’s a terrible fate to be a human in Nocturna.