missing stent

Ten days ago I had a kidney stone surgically removed.  At the time the urologist put a stent in my ureter.  After 7 days I went into his office to have it removed.  They first did KUB xray and then using the scope went in to retrieve the stent.  He could not find it in the ureter and said that it had moved up into the kidney.  I am now scheduled for surgery tomorrow so that he can get the stent out.  My question is how could this happen?  What caused the stent to go up instead of moving down?  What will he have to do in order to retrieve it from my kidney?