Don't worry, she saw you placing the egg under her! Put it under her wing carefully from the side, keep your hand round the egg to avoid her seeing it . Put a few chick crumbs down before you try this should distract her.
Leave the egg if she pushes it out she will likely put the egg back herself after you've gone away. As she is still sitting she is still wanting to hatch any eggs, that's good!
If you feel you must provide heat for the egg don't use direct heat, i.e. heat pad, use a heat lamp. The egg doesn't need water, in any form, this could prevent hatching as the chick will not be able to perforate the inner membrane. If the membrane is too damp the chicks beak will slide about and not go through the membrane. The egg will be ok for a few hours, keep it warm until you can get it in an incubator.
Careful if you try and take the shell off as it will cause bleeding if the membrane has not dried.
I would put the egg back under the hen, give the hen several small bits of bread and push any spread out nesting material back and leave her to get on with it.
i'll look back later. Have 4 hens with chicks, all laid away, been doing some juggling with eggs and chicks. One chicken hatched eggs on the garage roof, in ivy, chicks had to jump down! Freaks you first time you see it! Don't worry you'll manage well!
Gosh this is nerve wracking 
