I think you probably ment voulentary leave of absence(LOA) for surgery as opposed to layoff. In that situation you probably chose a certain amount of time that would be included in your LOA, this might have been a number you, your employer, state or county came up with, it varies from place to place and job to job. I would probably guess that you are an "At Will Employee" which means they can let you go at any time as well as you can tell them you are leaving whenever as well. Talk to you manager, let them know you just had surgery, you dont have to tell them what for, just let them know you arent quite ready yet. Keep in mind it is probably their right (If you arent under any defined contract) to say sorry, we no longer need you - this would be how it is in CA. Good Luck
Jeff
Not a lawyer - but has gone through a few sticky situations in the work place