Paper trading: My cash has disappeared

Today my cash has disappeared. I had 9000$ aprox of Stocks and 18000$ aprox of cash and now I only have my stocks.
It happened to me 2 weeks ago and the day after I had all my cash back.
It is a Paper Account, but I don’t know if this issue happen in Live Trading (if this had happened in Live Trading I would be very nervous).
Thanks.