Last night my budgie had either a night fright or she fell over. I did what I usually do, which is turn on the light and uncover her. When I did, I noticed she was on the side of her cage holding on with her beak but her feet kept slipping and she couldn't get a grip. I assumed she was just in shock and stayed up with her for about an hour. It didn't get any better, but she did eventually manage to get back up on one of her perches.
This morning, she's still having the same issue, she can stand still on her perch alright but her balance is off and she's unable to climb up her cage. I've suspected she has arthritis for a while now but she's never had issues like this before. I'm at a loss on what to do and I'd take her to the vet but due to Covid-19, that's not an option.
Make sure her food and water dishes are easily accessible for her.
If it's easier for her to be on the bottom of the cage until you get her to the vet then make the proper accommodations so she is comfortable.
I don't normally pipe in, but let me second FaeryBee's advice. If your local vet really isn't operating (ours is), I believe there used to be some vet resources on this site that you could do a remote session with. You could also try searching online—you might find something that way.
I've learned that just about any diagnosis I make is wrong. When something is out of the ordinary, don't assume you know the problem, go to a vet.
The last bird I had who lost motion in his legs had rapid kidney failure. Apparently, when this happens, days make a big difference and we lost him.
Have you examined her droppings carefully, looking for any changes?