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, specifically ones that are similar to TunnelBear, are disabled while trying to connect. The TunnelBear browser extension is designed to detect if a conflicting extension is present. However, if you discover that a specific extension is causing an issue with TunnelBear, try turning it off to confirm.
Clear your browser's cache and cookies
Clearing Cache and Cookies: Google Chrome
Clearing Cache and Cookies: Mozilla Firefox
Ensure that your browser allows cookies
The TunnelBear browser extension uses a standard cookie to ensure you’re authenticated on both the extension and website. Make sure that your browser isn’t configured to delete cookies automatically when exiting the browser. We also recommend making sure any third-party cleaner/privacy extensions aren’t set to automatically remove browser cookies. Lastly, ensure that cookies are enabled in the browser settings.
Enabling Cookies: Google Chrome
Enabling Cookies: Mozilla Firefox
Make sure that the TunnelBear desktop application is off
We recommend making sure that you aren't running the TunnelBear application while the TunnelBear browser extension is already on. Only one is to be connected at a time.