What happens to a bill that is voted down by the senate?

For a bill to become law it must be passed by both houses of Congress, so when the Senate passes a bill, the same bill must also go to the House of Representatives, or if the House has passed a similar bill, the two bills must be reconciled by a joint committee to produce a single bill that both houses can pass. Then when both houses have passed the same bill, the bill goes to the President for his signature. The President may or may not sign the bill, and if he doesn't, Congress can over-ride the veto if they have enough votes. Otherwise the bill dies.