I am wondering if she is in any pain? I think I would be asking the vet to check her over thoroughly. Including eyes, ears etc.
Otherwise she might have lost her confidence somehow. I have difficulty walking Brodie (a mixture of fear related issues and pure laziness) and one thing I have found is that it is best not to force her. It will only make her worse and the next day I will be lucky to get her out the garden

. You may need to build her confidence up slowly, with lots of praise and treats, and maybe take a favourite toy with you. Is there someone who has a confident dog that you can walk with? It could take a while and I think a behaviourist (one who uses only positive methods) may be your safest bet once the vet has (hopefully) ruled out any health issues.
Good luck
