The seriousness would depend on your symptoms such as
chest pain, shortness of breath, etc. and degree of blockage, number of blockages, location, etc. Sometimes there is unsatifactory response to medication and the condition requires a stent, and if stent cannot
be implanted due to occluded location, size ,etc. then a bypass would be the treatment.
I have occluded vessels for years and effectively treated with mediation...I feel well and no side effects, etc.