If she cannot walk, she may be egg bound. Is there a drake? Sometimes they can seriously injure a duck. If there is a drake, separate them.
Get her into a tub of lukewarm water so she can float. Do not leave her alone - we don't know if she will be okay in the water, but she might be. If she can float upright, let her - for an hour or more if possible.
If she is egg bound, the floating may help - and she will need extra calcium. Some people give a Tums for calcium, you can dissolve a calcium citrate pill and feed that with a tiny bit of bread just to get it down her, or mash some thawed frozen peas to get the calcium in there.
A good duck vet is always best, and I know we cannot always get to one.