Further details:

Symfony\Component\ErrorHandler\Error\
ClassNotFoundError
in /home/pbsc/public_html/templates/j51_bianca/error.php (line 12)
* @license GNU General Public License version 2 or later; see LICENSE.txt */defined('_JEXEC') or die;$app = JFactory::getApplication();$doc = JFactory::getDocument();$user = JFactory::getUser();$this->language = $doc->language;$this->direction = $doc->direction;
require()
in /home/pbsc/public_html/libraries/src/Document/HtmlDocument.php (line 731)
HtmlDocument->_loadTemplate('/home/pbsc/public_html/templates/j51_bianca', 'error.php')
in /home/pbsc/public_html/libraries/src/Document/HtmlDocument.php (line 785)
HtmlDocument->_fetchTemplate(array('template' => 'j51_bianca', 'directory' => '/home/pbsc/public_html/templates', 'debug' => true, 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
in /home/pbsc/public_html/libraries/src/Document/HtmlDocument.php (line 622)
HtmlDocument->parse(array('template' => 'j51_bianca', 'directory' => '/home/pbsc/public_html/templates', 'debug' => true, 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
in /home/pbsc/public_html/libraries/src/Document/HtmlDocument.php (line 640)
HtmlDocument->render(false, array('template' => 'j51_bianca', 'directory' => '/home/pbsc/public_html/templates', 'debug' => true, 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
in /home/pbsc/public_html/libraries/src/Document/ErrorDocument.php (line 139)
ErrorDocument->render(false, array('template' => 'j51_bianca', 'directory' => '/home/pbsc/public_html/templates', 'debug' => true, 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php'))
in /home/pbsc/public_html/libraries/src/Error/Renderer/HtmlRenderer.php (line 70)
HtmlRenderer->render(object(RuntimeException))
in /home/pbsc/public_html/libraries/src/Exception/ExceptionHandler.php (line 136)
ExceptionHandler::render(object(RuntimeException))
in /home/pbsc/public_html/libraries/src/Exception/ExceptionHandler.php (line 73)
ExceptionHandler::handleException(object(RuntimeException))
in /home/pbsc/public_html/libraries/src/Application/CMSApplication.php (line 334)
CMSApplication->execute()
in /home/pbsc/public_html/includes/app.php (line 58)
require_once('/home/pbsc/public_html/includes/app.php')
in /home/pbsc/public_html/index.php (line 73)
RuntimeException
Failed to start the session because headers have already been sent by "/home/pbsc/public_html/index.php" at line 2.

Also Joomla's template manager says we are on version 1.1.9 and that it is up to date, but according to my downloads page there is v1.1.10 available. However when I download and install the latest package it is still on 1.1.9 after doing so.

Never mind, the site had been compromised. But about that update issue... I was finally able to download and install v1.1.10, but why did it not show up as needing the update until I did it manually?