My computer turned off due to a power outage, so when I turn on the web version it no longer works, that is, it does not load, I have nextcloud installed on Ubuntu server, which files are damaged so that it does not allow me to access it via the web.
You are talking of a client or the server?
Without logfiles and other more specific information, it is difficult to help:
For file system checks, there is fsck. In case there are file system errors, it should also show up in the logs (/var/log/messages dmesg …)