what to do with chametz found after pessach which was not sold?

Shalom, Thank you for your question. The law of chametz that was owned by a Jew during Pessach is that after Pesach it is forbidden to be eaten by anyone (see Shulchan Aruch, Orech Haim, 448,3). So, the simple answer to your question is that one cannot benefit from the chametz at all, and it should be thrown out. However, it is worthwhile pointing out a few things. Firstly, if you did a sale of chametz, then even if you did not specifically have this chametz in mind (for example you found the chametz in a cupboard after pesach, and you had overlooked it and not put it with your chametz to be sold), then it was none the less sold to the non-Jew and is permitted after Pesach. Secondly, if you cleaned for Pesach, said the nullification of chametz and also did the traditional check for chametz, then if there is a large monetary loss involved, there may be room to be lenient and permit it after Pesach. Thirdly, this law only applies to real chametz – there are many products that we refrain from eating on Pesach because they do not have proper supervision, but they would not be forbidden after Pesach as they are not real chametz. In general, if there is a large monetary loss involved it is worth the effort to talk to a Rabbi in person to clarify all the details, because there are instances when the chametz will be permitted. But, if the amount of chametz is not worth a lot, and no sale was done, and the nullification and checking were not carried out (as we mentioned above), then the chametz should be thrown out. Blessings.