If possible, try another browser. That should give you an indication if the problem is with your browser or elsewhere. If another browser works you can use that as a temporary workaround to upload, and then go back to try to get your original browser working again.
If an alternate browser does not work, try another computer or phone, or another network to rule out other commonalities. If none of that changes things, the problem likely lies with eBay or your eBay account.
