It would be helpful if you could elaborate on the type of DNA (uncut/cut genomic, plasmid, fragment) you are working with to determine what the problem is.

Visualizing DNA in an agarose gel depends on:

1. The thickness/thinness of the gel. You can see smaller amounts of DNA in a thinner gel.

2. Amount of EtBr you are using & if you are adding it to the gel or staining/destaining after running the gel. If there is too much EtBr, the gel will appear too "pink" which doesn't help in visualizing DNA. A final concentration of 0.5mg/ml EtBr in the gel works well for most applications.

3. The size of the DNA. Larger/longer DNA incorporates more EtBr and is easier to see in smaller amounts. Smaller/shorter DNA incorporates less EtBr and is more difficult to see in small amounts.

From my experience, as little as 5 ng of linear DNA can be seen on EtBr stained gels. If 850 ng is not visible, I would strongly suspect that the measured value includes also RNA: if a DNA prep is contaminated with remnants of RNA (which may migrate as barely visible smear or not visible at all), it gives a high reading of both NUCLEIC ACIDs combined but the actual DNA concentration could be quite low. If enough DNA is required for downstream experiments, a new prep is the best way to go.

Is the ethidium stock solution good (dark red and not orange). Did you stain the gel for long enough time?

And as Libor Krásný points out, maybe your presumed DNA concentration is not correct.

One easy thing to try is to run a bit of DNA ladder on your gel. You should know the precise (and accurate) concentration of the DNA in the ladder bands from the spec sheet.