When I try to access my page I only get a blank page, not even an error message.
how can I get access again?
Or do I have to build everything new?
What version of Concrete are you running at the moment?
But I can't look it up because I can't sign in - nothing on the page.
If you have cPanel or ftp access you can see what version of concrete you are running.
If using V5.6 or earlier you can find it in concrete/config/version.php.
If 5.7 or later it is in application/config/generated_overrides.php.
I fear it is version 5.4
This may help narrow down the likely version it is on.
If you are on a voyage then there will be little you can do other than request that your web space provider put you back on PHP5.?
If you have cPanel access then it should be possible to rescue the site.
As @cmerritt notes, in the longer term, you will need to update the c5 installation to a core that is compatible with php7.2 (usually through intermediate steps) to either the latest c5.6.4 legacy, or c5 v8.5.x for current core.