Can you tell us on what system you installed your nextcloud (own webserver, webhoster, …) and which version?
It would also be helpful to tell us which error message showed up and if there are errors in your nextcloud logfile (can be downloaded from your admin-page).
https://nextcloud.com/changelog/#latest9 is the latest version, try this first. If you post logfiles, please only the one which are shown when you encounter the problem.