data: apply(matrix, makelist(makelist(random(100), 20), 20))$

find_value(a, x) := block(
   [p, q],
   [p, q]: matrix_size(a),
   catch(
      for i thru p do
         for j thru q do
            if a[i, j] = x then throw([i, j]),
      'not\ found
   )
)$

find_value(data, 100);
not found