My friend broke a jet on my old carb when we were cleaning it so we got a carb from a 2002 GS and fitted it in.  It fit fine just my throttle cable was a bit off, but we made it work with the new carb.  Now the bike won't even crank or start up.  Anyone have any ideas on what could be the problem?

Won't crank:

Check clutch switch, kill switch, key switch, neutral switch, sidestand switch, starter button.

Does anything work?  Headlight, horn, etc.?

Did u prime the carbs; meaning fill them with gas? 

If the bike won't crank, that's electrical ... has nothing to do with the carbs. Follow paulcet's checklist to see what you knocked loose or forgot to reconnect when installing the carbs.

If you meant that it is cranking but won't fire, average is probably on the right track .. no fuel in the carbs. Put the fuel petcock in the PRIME position to make sure you get fuel into the float bowls. Once it has started and run for a short period, you can put it back to the ON position and let the vacuum operate it like normal.

...ken...

fuel spark compression brodude