It depends on how deep the hallow, and what sort of area we are talking about, but without digging up turves, you can put a thin layer of topsoil over the area until the grass grows through, and continue this way until you reach the level

Fill in the hollow, tread it down lightly, unless its a very deep hollow, when you should walk over it, level it and then rake up the top again to make a friable tilth - sow grass seed NOW - it'll grow quickly at the moment, but leave it a week or two and it'll take ages...