First of all, in the case of all mechanical clocks, they require service every 8 to 12 years. Since this NEVER happens, the clock will run for decades until metal-on-metal wear causes it to fail, sometimes spectacularly. If the power of a wound mainspring is unleashed suddenly, as is the case with a broken wheel tooth or pinion tooth/stave, the backlash can destroy the clock movement.
Basically, for a mass produced clock this is game over, unless you want to spend an absurd amount of money rebuilding a wrecked movt.
For mantel clocks, because they are mostly portable, they are dropped off and serviced at your friendly neighbourhood clockmaker.
An Overhaul entails complete disassembly and cleaning in Industry Standard solutions, as well as hand cleaning of certain components. Mainsprings are assessed for wear and loss of modulus of elasticity, and replaced when necessary. The pivots on all wheel are polished and work hardened, and the plates are bushed to restore the precise geometry needed for optimum power transfer. The clock is now re-assembled and lubricated, and test run for regulation and timing. This includes not only rate adjustments, but also beat error correction and oftentimes in weight driven clocks, we find that the weights have been modified, so they need to be restored or repaired as well.
In the case of Floor clocks or Grandfather clocks, the mechanism is picked up and taken to the machine shop for overhaul. There is no way to perform the work in situ.
This brings me to the next point, if your long case or GrandFather clock is being serviced in the home, it is NOT being overhauled, usually this falls into the 'Quick Fix' category.
So what are examples of 'Quick Fixes?'
Lubrication, which should be done every 4-6 years, can be done in situ, but the dial and hands MUST be removed, and often the mechanism must be removed from the case. If the dial and hands did NOT come off, your clock was NOT lubricated correctly.
A complete lubrication requires the removal of the dial and hands, but a partial lubrication might only entail the escapement being done.
If the clock does not perform properly after a lubrication, it is usually the sign that severe wear and tear have made an overhaul necessary.
Other 'Quick Fixes' include regulation, Beat Error correction, Chime adjustment, and timing and regulation. Sometimes the chains get pulled right off the sprockets or they get jammed. Humidity changes can warp clock cases and sometimes you can correct for this by re-positioning the movement in the case wherever possible.
I've come across quite a few recently that have been absolutely butchered in situ, and I do spend a lot of time correcting previous 'repairs' by 'Unskilled Hands,' shall we say.
Anyway, we are the custodians of these often antique time machines, and they are one of the few actual antiques that can be still used on a daily basis in this increasingly tech oriented world.