I wouldn't worry about the economy mode message, it's just because the car has been standing. Just start as normal and after you are able to drive for a while the battery will get charged enough to cancel that message. Just means the system has turned off certain things because the battery is a bit low.
Although a 5 year old battery could require changing so you'll have to see after you have sorted the binding brake.
Previous Citroens, GS Hatch, GS Estate, DS Special, GSA Hatch, BX, Xantia & C4 1.6 Petrol 05 SX Hatchback. Now DS4 Prestige 210.(2016)