Lumia 1020 :: Why Have Entries In Calendar Disappeared From Phone And How To Get Them Back

I use the daily calendar page to record vehicle usage. business activities, invoiceable information and a host of other items and have done since I purchased my first Lumia 920 when they first came out in Australia. I invoice on a monthly basis and by necessity at times need to refer back to information sometimes months old. Yesterday I tried to retrieve information from early September and everything prior to the 19th of that month is gone. I am lost without this information. I am not teenage savvy on the idiosyncrasies of smart phones but I would have thought that if the availability of an option to record past events in calendar existed then those entries would have been sacrosanct, subject only to loss with the loss or damage of the handpiece and even then I thought they saved to the cloud. I've searched the forum as best I know how and haven't found an answer to the problem or any indication of how to retrieve the information.