From the FullForm you have seen that your expression is essentially Grid@List@List.
To work with the values you need to remove the Grid, probably best by using the Part function.
E.g. to compute the determinant of a random 4x4 matrix placed in a grid with some divider lines:
matrixInGrid = Grid[RandomReal[1, {4, 4}],
Dividers -> {{False, True, {False}}, {True, True, {False}}}];
matrix = matrixInGrid[[1]];
Det@matrix
Hope this helps.