I created a survey through lime survey on Saturday, but by yesterday I could no longer access the link:
demo.limesurvey.org/index.php?r=survey/index&sid=389788&lang=en

Please, I am new to Lime Survey, what do I do to find it again?

The database of the demo installation is wiped on a regular basis (I think every 24h), so I am not surprised that you can't find your survey.

The demo server is just for a first try of Limesurvey. If you want to do more with it, you need to install it on a webserver or contract a Limesurvey hosting service like Limeservice.com.