Your question is not very much clear, but try this:
if(ddl.selctedindex>0)
{
combobox1.enable=false;
}
else
{
combobox1.enable=true;
}
In case you still face issue, then enable Javascript debugging in your IE and then put "
debugger;
" at the start of this method. See how the execution is happening and if something is unexpected.