I have one important document in Libre and I cannot open the program
I wouldn’t be on this screen if I could answer my question. I am totally frustrated by this site.
I am not Mr. Libreoffice and not aunty Documentfoundation. Many contributors here do not exactly love this site. How would you advise me? Shall I cease to contribute?
(This is based on the assumption that you still have your document in a loadable file. If there is damage on the document level a different approach may be necessary.)
You can try to reset your user profile. Besides settings templates and “macros” are also saved there. Be cautious if you have something of that kind.
See this wiki page for more detail.
You may also unistall LibO completely by system tools and install a version of LibO anew.