FIrst stop using turns.
Now disband any fleet you have and sell everything you can on the market.
Tentatively use a single turn to see if you are starting to come out of debt.
If not try assigning more land on your planets to commercial (assuming your terran) then try using a turn again.
If you are still simply going further into debt I suggest you restart.
There ARE artifacts that give you small amounts of money and others that could take percentages away from your debt, but its usually best if you just restart your empire at that point.
Good luck.