Ask a real estate pro: The association says I missed payments. I say I didn’t. Now what?

Source: Sun Sentinel — 

Q: For the last several months, my condo association has accused me of missing two payments from several years ago that they apparently just discovered. I paid my dues with online bill pay and provided the association with proof of payment for those two months. The property manager is still asking for the “missed” payments and refuses to give me a copy of my account ledger. What can I do? — Ira