Some possible things to try:
If you are using a VPN or a hotspot, or privacy software like Ghostery or Disconnect, try not using those to see if that makes any difference for you.
Try using a different link to reach eBay to rule out a bad link.
Try another browser to rule out a browser issue.
Try another device to rule out a device issue. I had a problem signing in that turned out to be due to a flaky keyboard -- turns out the shift key only worked sporadically with one particular key that happened to be part of my password.
Try another network to rule out a network issue -- such as using the eBay app when not connected to your usual WiFi.
If you have another eBay account, see if you can sign in to that account or if you have problems there. Maybe the issue is with your account itself. Check the account details closely if you can sign in to make sure nothing has been changed -- particularly contact info and addresses.
If you have your eBay account linked to a social media account already, try signing in that way instead of with username and password. If the social media account is not linked this may create a new eBay account, though.
If you are able to sign in here to the eBay community successfully, see if you can reach your eBay account using the My eBay link you can reach by clicking on your icon picture placeholder in the upper right of most community pages.
See if you can sign in to your account on ebay.ca instead of ebay.com.
If none of that helps, use the "Need help signing in?" link on the sign-in page where you are prompted for your password. Select the "reset" option, but instead of resetting your password, use the contact link on that page. That is specifically for users that cannot sign in.
