Resection. Find two or preferably three distinct terrain features (natural or man-made) that you can also identify on your map. Take a back azimuth from each feature, adjust for declination, and draw the lines from the features on your map along the back azimuth. Where the lines intersect is where you are.
If you're somewhere with no terrain features, you're screwed.
I don't know what a back azimuth is without looking it up, but you explained it in a way that I was able to figure out how you would do what you explained. Thanks!