Before Albert Einstein came up with his equation E=mc^2, the scientific world believed that mass and energy were two independent phenomena in the universe. However certain occurrences, such as radioactive emissions could not be explained by this divide between mass and energy. Einstein worked out the equation by assuming the speed of light was constant to provide the link which would later explain how stars worked as well.
Prior to Einstein’s famous equation, the scientific world viewed mass and energy as separate entities that could not be interconverted. Scientists also viewed distance and time as constants, and thereby speed as a variable, including the speed of light. These theories in science explained most natural phenomena, but not all. There were some exceptions that did not fit these guidelines but were largely ignored until Einstein provided the link between mass and energy. For example, Marie Curie was unable to explain why certain radioactive isotopes emitted energy for years on end without running out of it, when there seemed to be no obvious source of energy.
Einstein worked on this conundrum for almost a decade before he came up with E=mc^2. He assumed the speed of light to be constant to an observer who is at rest relative to Earth, and this assumption (which later became a standard law in physics) is what helped Einstein link mass to energy.