$N$2 - the reference remains always to $N$2 (e.g. you copy formula from cell X2 to Y3);
N2 - both column and row references change (e.g. you copy from X2 to Y3 and you get the reference to M3);
$N2 - only row reference does change (e.g. you copy from X2 to Y3 and you get the reference to $N3);
N$2 - only column reference does change (e.g. you copy from X2 to Y3 and you get the reference to M$2).

What you mean by Drag and Drop? To copy formulas:
a) You copy formula, and paste it to any range - references are changed according to rules above;
b) You hover mouse cursor over right lower edge of cell until a cross appears, and then click on this corner and then drag cursor up/down/left/right keeping left mouse button down - references are changed according to rules above;
c) You activate edit mode for cell with formula, copy the formula from formula bar, press Esc to leave formula bar, activate edit mode for target cell, paste the formula into formula bar, and press Enter - references remain same as in source cell.

From your post, I get feeling the formula in target cell(s) refers to wrong cell. This may be easily happen, when you copy formulas to range up/left of source cell. E.g. when the formula in cell y10 refers to cell A1, and you copy it into cell in column Y or row 9, then it will refer to non-existing cell.