Zombucks, coins, and other items never disappear in the game. Most likely, you have bought an expansion on the Big Land or some useful items, tools (super-swords, bangs), or just skipped some tasks.
If you are absolutely sure that something has vanished, please specify what exactly was lost, the amount of it and the date (or at least a 7-day period) when they went missing. We will find the info that explains what your zombucks have been used on and where did your items go.