As has already been noted there are different types of manhole lifters, most can be bought from builders merchants for around 10- £15.
You may get away by using a thin cold chisel, placed carefully under one end, but some Mine exploration groups, modify the manhole in some way, so as to incorporate a locking system.
We had one on our manhole at East Greenlaws mine, but unfortunately, like some of these ideas, jammed solid and we had to get in by breaking the manhole cover.
Some of these are made of cast iron which does not like being hit with a large sledge hammer!