Comparing two columns of data and pasting in data from third column...

I am *such* a novice at excel and am about ready to cry with frustration!

I have two sheets in one workbook - and want to compare Column E in Sheet 1 to Column A in sheet 2. IF they match, I need the data from Column D in sheet 2 to copy into Column G on Sheet 1. Does that make sense?

I've been using this formula: =VLOOKUP(E668, Sheet2!A3:K6701, 4), which I've entered into Column G. And it WORKS for that *one* cell. BUT when i try to cut & paste it down the entire Column G, the reference range "Sheet2!A3:K6701" keeps changing its numbers OR becomes "REF and I need it stay the same. What am I doing wrong???

I'm a moron.... My moronic thanks to anyone who can hep me!

thank you so much!!