You activated a buggy PHP-Nuke block. Now, your site is broken and you get a blank page, or you only see errors and the administration panel is unreachable because
the block breaks your page's design...
There is a very simple solution to this very annoying problem (see Site crashed when block was created): just
replace the broken block, say block-Broken.php, with a script that does not contain anything but the following line:
<?php $content=""; ?>
Then, the administration panel should be reachable and you can deactivate the offending block.