I assume the ship is also owned by you? Beyond that I am not sure what else the problem could be. Double check that you have builders and not workings. Make sure that you are resuming the right station.
Sounds for me that there are no builders assigned anymore to the shipyard (maybe fired or dissolved when the previous owner died/faction dissolved) while the yard were under construction).
Try to take the builder into your party and enter the exit room.
Choose (if possible) via the drop down menu of "Party menu" "add to construction".
If you an add builders to the station construction site, you can re-start it via the inentory resume-button.
If you cannot add builders to the station construction site, you need an admin.
Maybe they can help ...
Worst case would be aborting the construction.