Hello What do you need help with? Staff members are always online and we all can help you. Also if you need immediate help join the Mcc Community Discord (https://discord.gg/9jzwQDQ) and ping staff members.
That means the session is invalid, do you by chance have someone else using the same account as you? If not, I would try to sign out and back into the launcher. Also, the account has to be a premium Minecraft Account.
Are you using a free MC account? Because you need to buy your account to actually join servers. If you have, and have relaunched your game, I suggest re-installing your Minecraft files, as there could have been an error, and when downloading it, do it off the official website! Have a great day!