What should I do now concerning the fetus that my wife miscarried in the fifth month, which remained alive for five hours then died, and was neither washed nor prayed over, and was buried in an unknown location? And do I bear a sin because of that?

If a miscarriage comes out alive and cries out, it is obligatory to wash it and pray over it by consensus. The same applies if it does not cry out but has completed four months, according to Ahmad. It is permissible to pray over a grave for a period of one month after its burial. The evidence for this is the action of the Prophet, peace and blessings be upon him, when he prayed over a fresh grave, and his prayer over the woman who used to clean the mosque after she was buried. If it is not possible to stand by the grave, then only seeking forgiveness and repentance is necessary.