external monitor no signal

I have a Macbook pro 16 of 2019, working great with the lid closed using an external monitor. Suddenly one day when I close the lid my external monitor gets black and says "no signal input" if I open the lid, the cover... everything works great both on mirror display and as extension... but i prefer to work with my lid closed on one screen only. is there any solution for this??? i use Catalina and don't want to upgrade to big sur... thank you