In general, an elevated BUN/creatinine ratio indicates a "prerenal" problem, meaning the kidneys themselves are not diseased but there is decreased blood flow to the kidney. This can be a result of many different things, like low blood volume, low blood pressure, renal artery stenosis (narrowing of the artery supplying the kidney) and others.
With kidney disease, you would see a *low* BUN/creatinine ratio.