~snip~
Obviously your 20k Bitcoin will sold immediately and you will loose 18k of your fund instantly . Already many have explained in above about it , that because of your lower sell order . Don't make this type of mistake otherwise you have to face loss and you will never get your fund back.
This might not be the case. If you sold at a lower price, all the buy offer from $20k below will be filled and losing $18k might not be the correct amount. You will surely lose a lot of money but not to the extent of losing $18k. Remember there's a support level which limit the seller from dumping their Bitcoin easily, technically $2k price might not happen not unless you sold a lot of you Bitcoin but with this case, it will not affect that much.