We wouldn’t say it that way but “The bed was slept on by someone” works.

"The bed was slept on," and "the bed was slept on by somebody" are examples of prepositional passive constructions. More information: https://en.wikipedia.org/wiki/Passive_voice [excerpt] If the agent is mentioned, it usually appears in a prepositional phrase introduced by the preposition by. Without agent: The paper was marked. With agent: The paper was marked by Mr. Tan. The subject of the passive voice usually corresponds to the direct object of the corresponding active-voice formulation (as in the above examples), but English also allows passive constructions in which the subject corresponds to an indirect object or preposition complement: We were given tickets. (subject we corresponds to the indirect object of give) Tim was operated on yesterday. (subject Tim corresponds to the complement of the preposition on) In sentences of the second type, a stranded preposition is left. This is called the prepositional passive or pseudo-passive (although the latter term can also be used with other meanings).

Im a native English speaker (UK) Im not a teacher ...but "someone slept on the bed " and Re Tim's suggestion "the bed was slept on by someone" are correct but "on the bed was slept someone" is incorrect .. Perhaps you are trying to say "someone was asleep on the bed" ...