Storing it in those temps shouldn't hurt the synth. However, sudden temperature changes aren't too good for electronics because of thermal expansion/contraction and the possibility of moisture forming inside the unit due to condensation. If you have it at those temps (25F is -4C, 5F is -15C - conversion for my own benefit..), do not switch it on until it has acclimated to the temperature in the room, assuming that you'll heat the room when working there. It's hard to say exactly how long you should wait, but when it's no longer very/icy cold to the touch, it should be safe to switch it on - could be an hour, or it could be more or less - you'll just have to use your judgement.
