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Three months is too long to go without driving, the battery will be dead-
But in any case- when you disconnect or discharge the battery, the steering will have a warning. You just need to drive it and perform a 90 degree turn to both the left and to the right. There is no coding necessary. In extreme cases, you may need to perform the basic settings for the system.
With a fault like that, I would first check and clean up any connectors that may have oxidised when not in use, no electrical flow. Put dielectric grease on the fittings before reconnecting them.
Also, given the time it has been sitting, I would look for signs that mice have been in the engine bay, and may have chewed some wiring. They will often build a nest, and if wires are in the nest area will chew them down to bare wire and sever them or cause them to short to body panels or the engine block.