In your gmail webmail account have you selected to use the 'insecure apps' option?

Do you use the two step authentication to access gmail using a phone?

Is this a POP or IMAP mail account?

If you updated to the recent Thunderbird version off 38.2.0 and all your mail accounts cannot send or receive, then check your anti-virus/firewall program to make sure Thunderbird is allowed. On one occassion, I had to block and then reset to allow. It seemed that 'norton' thought it could only allow a previous version.

If you set 'Authentication type' to 'OAuth2' in your Thunderbird account settings for your Gmail account you don't need to allow 'less secure apps' any longer.