Multilayered boards normally use the sandwiched hidden layers for ground plane connections (normally the zero volt and 5 volt power supply rails in a classic 4 layer PC Motherboard) rather than for circuit traces which normally occupy the top and bottom surface layers using through plated holes either as 'vias' or connections to the zero volt and power supply voltage(s) groundplanes.
Modern motherboards might well have 3 internal groundplanes (0v, +5v and
+3.3v) with the less commonly used +/-12v rails using surface tracks. Less complex circuit boards may well manage with just one groundplane (usually the 0v rail) or even none. With modern mass produced electronic gadgets, the PCB can be the most expensive item in the BoM list, especially more likely if it is a multilayer board with an internal groundplane or three. Afaik, only military grade kit used internal layers for signal connection traces.For damaged through plated holes used for vias, it's usually reasonably easy to effect a repair. When it comes to open circuit through plated connections to an internal groundplane, you simply locate the nearest handy component pin connected to the groundplane (0V, 5v or whatever) and solder a wire strap to bridge the connection.
Soldering a wire strap from a handy 5 volt pin to the input tag on the
3.3v analogue regulators used to power 3.3v 80486 CPUs was a common enough remedy to deal with fatigue stress cracking of the through plated connection to the internal 5v groundplane induced by thermal cycling from the heat of the regulator which itself was able to cope (but, contrary to expectations, not the PCB it was mounted onto).