I just answered a question on another blog about "fallen arches" are decided this was a topic that needs to be addressed - even if I have done that before!
Fallen arches are not normal. Usually it happens when the tendon that holds and supports the arch (the posterior tibial tendon) weakens or tears. This tendon can weaken with age, weight gain or flat feet. The problem is very subtle at first and may just present itself as subtle aching of the inside ankle and/or arch.
If diagnosed early on, simply supporting the tendon and the arch with custom orthotics is usually all that is needed; but if the tendon continues to be stressed without support it can tear or even rupture.
I currently have a patient who's tendon partially tore during her second pregnancy and she had been suffering for year until it finally ruptured.
Sometimes physical therapy is helpful, but if the tendon has tears and scar tissue, surgery is generally needed. The good news is that people do GREAT after repair of the tendon. Actually, that is the case with any tendon. If there is prolonged pain, there is always pathology. Once surgery addresses the pathology (tears or scar tissue) the person does great.
The only other consideration is what to do to stop the stress on the tendon after it is healed. Custom orthotics are used but it is also possible to have a stent inserted into the joint that supports the arch for a more permanent type of support. This surgery is a simple procedure that should be considered with this problem.