most likely fleet upkeep put you into debt
* Stop using turns
Using turns will ensure you keep increasing your debt through both upkeep on ships and debt interest
* Disband all fleet
As above stated ships require upkeep per turn, watch your future spending by clicking on income and seeing where you spend.
* Sell items on market
Sell enough items to bring you cash up above back into positive.
Check important events to tell if things are sold
This is so you don`t need to use turns you can refresh your events page to keep an eye on items being sold, can also click on important events and click on refresh (f5) for this
* When they sell log out and back in
The logging in and out will refresh you figures correctly.
some tips are here
http://hhunters.gfx-pro.net/hellhunters_aug_2006/GC_WebPages/GC_FAQs/GC_FAQs.htm