When my 2000 ford focus ran out of gas. why wont it start?

Most likely your fuel filter is clogged from sediment in tank or fuel pump is faulty, to narrow down that fuel pump is not faulty try turning key to on position DON'T START and listen for a soft humming sound from rear of car. If no sound is heard you likely have a bad fuel pump. You may also try pushing on schraeder valve on fuel rail (looks like tire stem for airing tires only on fuel rail, probably has black cap on it like tires or similar) if gas squirts out you are getting gas to engine and must diagnose other problems. Good luck.