Hello Krisztina

First of all update to latest version 17.1.2

If you still can't open the project then there is something wrong.

Then select the object in the project selection, just select  it, don't double click.

Then use the backup button and hope there is an automatic backup that works, then continue with that one.

It is always importand to make regular backups with export project in step5, x5 reminds you of this automaticly, do this after some time or after a lot of changes so you have always a project to faal back on. If you ave done this, then use your previous backup from step5.

goodluck.

https://help.websitex5.com/en/v17/pro/gestione_backup.htm