If you're not able to resolve issues with the extension with a fresh install, try the following.
Disable any third party extensions
Make sure other extensions, especially ones that provide a similar function to TunnelBear, are disabled while trying to connect. The TunnelBear browser extension is designed to detect if a conflicting extension is present in the majority of cases. However if you discover that a specific extension is causing an issues with TunnelBear, try turning it off to confirm. Then please let us know.
Clear your browser's cache and cookies
Ensure that your browser allows cookies
The TunnelBear browser extension uses a standard cookie to ensure that you are authenticated throughout both the extension and website. Please make sure that your browser is not configured to delete cookies automatically when exiting the browser. Additionally, please ensure that any third party cleaner/privacy extensions are not set to automatically remove browser cookies. Lastly, ensure that cookies are enabled in the browser settings.
Make sure that the TunnelBear desktop application is off
Please make sure that you aren't running the TunnelBear application while the TunnelBear browser extension is already on. Only one is designed to be connected at a time.