Not every window problem means you need a full replacement, but a few signs reliably point in that direction rather than toward a simple repair.
Condensation forming between the panes of a double-pane window means the seal has failed and the insulating gas between the panes has escaped — this can’t be repaired, only replaced, since the insulating value is permanently lost. Windows that are difficult to open, close, or lock, or that let in a noticeable draft even when fully closed, usually indicate frame warping or deterioration rather than something fixable with weatherstripping alone. Visible rot in a wood frame, or noticeable difference in indoor temperature near the window compared to the rest of the room, are both signs worth taking seriously.
On the other hand, a single cracked pane, a broken latch, or minor drafts around otherwise sound frames are usually repairable without replacing the whole unit. If you’re not sure which category your windows fall into, it’s worth getting an honest opinion rather than assuming the worst — or the best.