How can I recover lost folders in Gmail?

I've just realised a few folders that I set up in gmail, are no longer there. There was a lot of highly sentimental emails, along with important ones, in there and I'm devastated. I backup my laptop with an external hard drive, but I'm not sure if they back up emails as well - can't seem to find anything (clearly not overly techo!). Can anyone please help?

[Re-Titled by Moderator]

Original Title: Lost foIders in gmail