It can be several different things. Does the bike idle correctly? Does it rev up when you turn it the little amount that you can?
Did you change the cables? If so, did you make sure that the "throw" is the correct length? (that's where the cables wrap around the throttle body) if they are not the correct length they may be almost stretched to their limit.
You might simply need to adjust the free play. There are adjustment nuts at the handlebars/throttle grip. If they are to tight you could have taken all the slack out of the cables.
The cable could be bad. If it has rusted inside an older cable it could be stuck. You could rey spraying WD40 in there and waiting a bit to see if it loosens the cable up.
These are just some real quick things you can check. Without actually seeing the bike I couldn't tell you exactly whats wrong. If you can provide me with more information I might be able to go more deeply into your problem.
Hope this helps...
