Rambeer, who hailed from Siwana village here, had been lodged in the jail since January this year in connection with a murder case, they said.
The undertrial was found missing by jail officials while the prisoners were being counted, they said, adding, he was later found unconscious in his barrack.
Rambeer was declared dead by a doctor, who was rushed to the prison by the jail officials.
A postmortem examination, which was conducted at the civil hospital in the presence of the Chief Judicial Magistrate, revealed that he had died of cardiac arrest.