California condo, no cable signal, who should fix the problem?

Recently I lost cable signal on a line coming to my unit from next door condo, and serving both units (daisy chain).
Next door unit was sold before I noticed signal loss.
It turned out that the prevous owner decided to replace multiple wall outlets on that line with blank plates; he considered another line he had sufficient. It is pretty clear that the problem is behind one or more of those new blank plates.
Coax cable goes inside internal and external walls of the next door condo before entering my walls.
New owner is not affected by the problem and shows no interest to address it and denies access to cable company technician.
1984 HOA CC&Rs define “central television installation” as part of common area.

Who should fix this problem? What bears more weight - the fact that the problem was generated and may only be resolved inside known unit, or the fact that cable tv is common area/property? Is it the new owner or HOA?

As others have said, contact the cable company.
Have them resolve it.

I would ask the HOA to fix it just like I would any other element that passes through a unit behind the walls like a common drain pipe or vents, etc.