1. Create a Backup of Your WordPress Site

Before commencing any troubleshooting you must always backup your site especially your database. From your cPanel, click on phpMyAdmin. Select the database for your website and click 'Export'. 

You now have a copy of the website's database which is key in retaining your content. 

2. Switch Off Your Plugins

Especially the most recent ones. 

After each plugin that you deactivate, reload the site. If the site starts working you will know which plugin was problematic. You should upgrade or delete that plugin. 

3. Switch your theme to default

On your dashboard go to Appearance>Themes page and then click on the Activate button next to a default theme.