Unfortunately, there is no camera function that will reset the mirror if it is stuck due to either the spring or other secondary component corroding bring out of alignment . The long idle time, depending on how the camera was stored may have damaged the mirror mechanism. Gently, touching it only on the edge, try to move the mirror. Do this in a clean area with hands as dust free as possible. You can also use a wooden or plastic toothpick so that your hands do not touch the glass. If the mirror cannot be freed up to lift and return properly, the camera will require professional service to repair the sub-mirror assembly.
Please keep in mind that my diagnosis & solutions provided are directly dependent on the accuracy of your description of the problem. As with any "do it yourself" fixes, success is a "team effort", since I can't see or touch the camera, and relies on the customer's manual dexterity and ability to follow the instructions well.
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