There are two different types of saved progress for Free Code Camp: your profile and your browser cache.
A list of your completed challenges is saved to your account in the FCC database. You can see the list of completed challenges by looking at your public portfolio. With a growing curriculum already over 1,400 lessons and a growing user base several times that size, FCC does not store every solution to every challenge in its database. When you complete a challenge, there is a popup that gives you the option to download your solution. This gives you the option to save copy of any solution that you may want to reference later. There are some challenges which are classified as projects required for certifications. Your solutions to those can be viewed on your settings page.
Your in-editor code is saved in your browser’s local storage. Recent in-progress code from the challenge editor is also saved in your local browser cache when you run tests. If you are completing lessons and do not see your recent code, then your local storage has been cleared or something is preventing FCC from writing to your browser’s storage. This could be a browser setting, a privacy extension, or a browser version incompatibility. Especially as you get to more complicated challenges that may take multiple sessions, I strongly recommend saving your in-progress work outside of the browser cache.
This is a good opportunity to learn the ins and outs of your GitHub account, but you can also just save locally or use a service like repl.it which allows for versioning.