dental help-breaces problem?!

o.k so i have a little gap in the front of my teeth,it is barely noticable,and the last time i went to the dentist,he said it would take about 6 months to fix with braces.this was about a year ago.last week,a tooth from the bottom left side (almost the back of my teeth) of my mouth fell off.if i were to get braces,do i have to get that tooth replaced,before getting braces?can a person only get braces on the top of their teeth iuf they wanted?need to know,thanx.