The major decision that needs to be made is whether intervention is necessary. This decision can really only be made by your cardiologist after he/she has a chance to sit down with you to review your symptoms, clinically examine you and review your latest angiogram.
Your physician might opt to ensure that you are on good medication therapy and see how you do with this. Generally speaking, if you are quite symptomatic and depending on where the block is and how severe it is, it might be possible to open the vessel with angioplasty.
Repeat bypass surgery might also be considered if you are very symptomatic despite a good medication regimen and the block cannot be fixed with angioplasty.
Your Cardiologist will have to have a careful look at your latest Angiogram and make some important decisions based on the current situation. Are your symptoms bad? has the closure of this vessel completely stopped blood flow to a portion of your heart muscle? Angioplasty has moved on a lot in 16 years, can they re-open the original blockage for which that graft was done?
I doubt if they can do anything with the closed vessel, if it's a vein they just collapse totally usually when they give up. If the graft is an artery perhaps it can be stented. There are lots of options but it really depends on many things which only your cardiologist has the information for.