Originally when website was changed from developer to production mode this error was occuring;
There has been an error processing your request
Exception printing is disabled by default for security reasons.
Error log record number:
But some areas of the website were accessible but now the CSS is not loading at all.
Any help would be grateful.
I guess you've tried, but if you haven't - you should deploy static assets:
php bin/magento setup:static-content:deploy
php bin/magento cache:flush
Also, it could be due to wrong permissions. In that case, you can try with:
chmod 755 -R pub/static