Project becomes 404 after `git push -f`

In my private project, I squashed all old commits, and did git push -f. Then I got 404 Not Found when I open this project on <gitlab.com>. This project still exists in my project list, and I will get a name already existing error if I create try to create a project with the same name. I also try to clone this project using git clone, but got

warning: You appear to have cloned an empty repository.