after "combine" unconnected lines wont fill with color

sorry if this has been asked a million times. I am so novice I don't even know how formulate a coherent question enough to search for an answer to see if this has been addressed already or not.

so say I draw a circle. than inside of that circle I draw a semicircle. once I combine these two layers and try to fill it in with a color, the semicircle creates an imaginary line from the one end node to the other that wont fill with color. how can I prevent this?

I am trying to do a line drawing. Once im done drawing all the lines, im trying to layer it into one element to then fill shade ect. Yet with the afore mentioned problem it wont weld or combine into one curve or path so I can color it accordingly. it always has these random shapes inside the design that wont fill.

thank you for your expertise.