Themken: For a while customers from USA had a lot of problems. That was a few years ago.
These problems were of a complete different nature (at least when we are not talking about different things). It was mostly US banks or credit card companies that didn't allow payments to Cyprus for their customers. So some people weren't able to buy stuff.
But I haven't seen any threads claiming that GOG just deleted the wallet funds (except for funds from the fair price package that expired after one year).
My guess: Maybe the OP added funds in their local currency and after that switched the currency settings to US$. In that case the funds would still be there, but invisible as long as they doesn't switch back to the original currency.