If you don't want to use the specific FPlus tool, you can use a standard cookie manager. Install from the Chrome Web Store. Go to Facebook.com and ensure you are logged in.
Cookies expire. If FPlus stops working, the first thing you should do is refresh your Facebook tab and grab a new cookie. Troubleshooting "Invalid Cookie" Errors If FPlus says your cookie is invalid, check the following: Get Cookie For Fplus Chrome Extension
Open Facebook.com. If it asks for a password, your session expired. If you don't want to use the specific