Typically gases such as nitrogen or carbon dioxide are used to remove oxygen from a confined space and render it inert. Often an area is made inert in order to remove the potential for fire, or to stop oxidation of a product within a space and keep it from spoiling. If you are attempting a rescue, or have other need of entry, you must be able to anticipate what changes may result within the space if you return an oxygenated, breathable atmosphere to the area.
To do this, you must know why a confined space has been inerted in the first place. Certain products such as organic fertilizer can be subject to self-heating and spontaneous combustion. If you were to start up your ventilation, and return a breathable air to the space, you may be creating a confined space with a fire burning inside. It's unlikely this is what you intended when you were trying to create an atmosphere suitable for entry.
When attempting to overcome an inert environment with the plan of entering the space, the best way to protect responders or workers is to know precisely what the inert space was designed to protect. Sometimes expensive switching gear or other costly equipment is in a space and inert gases are used to prevent water damage in case of a fire.
Whatever the reason for the inerting, you must understand what is being protected, and what may result if you shut down that system. By knowing what is being achieved by administering the inert gas, you will have a better idea of how to keep control of your confined space entry.