If this re-investigation is pending on election day, it could cost Clinton a point or two, just by the fact it’s pending. How often have we had a candidate for POTUS being investigated for criminal activity?
A lot more depends on why the re-opening and what comes out between now and election day.
It is still highly unlikely that she’ll be indicted, but this cannot be good news for her campaign.
They can make the same kind of argument with Trump’s ongoing irs investigation. Not the same thing but they’d be able to spin it that way.