Assuming it is a reasonable quality piano that up to now has been regularly maintained, and will being kept in a reasonable environment, it shouldn't come to any harm if it is not going to be played for a couple of years. Make sure the room is not too hot, and that the piano is not standing in a sunny window.
A piano may be untunable if:
1) the strings are so old and rusty that they keep breaking. Solution - restring piano. Not cheap
2) the wrest pins are too loose - restringing with new pins may help but if the wrest plank has dried out so much that it has cracked, nothing can be done with an upright piano. You can replace the plank on a grand but it is very expensive and may not be worth it.
3) the bridge is so badly cracked at the bridge pins that they can't withstand the string tension. May be repairable but very expensive. Note that many bridges have small surface cracks that are nothing to worry about.
4) the frame is cracked. Theoretically the frame can be replaced or even welded but usually cheaper to replace the piano....
A modicum of common sense should avoid all these problems!