Yes, sash and case windows can, in fact, be repaired. How complicated the repair will be depends on what exactly is wrong in the first place. In cases where damage isn't too significant, you can even do the repairs on your own.
For instance, if a sash window happens to have a broken pane of glass, this can be easily repaired without calling in a specialist. Also if the window happens to have very localized wood damage, it will also be quite straightforward to repair on your own.
However, when damages are too significant and not very localized, proper repair becomes a lot more technical, and will most likely require having to call in a sash and case window repair expert.
Also, for what it's worth, most damages to sash and case windows are very rarely localized so the chances of having to call an expert is pretty high.
