How old is your cockatiel?
Did you see your cockatiel eat recently?
If the crop is not empty for a day. You may have crop problem, could be slow crop or impacted crop.
The treatment can be complicate. If you have syringe and silicon tuber, you can fill the crop with warm water, and empty all seeds out. The best is to visit vet for such situation if possible. And, keep your bird warm!