There is nothing more infuriating than the white screen of death with the word 'error' written int he top left hand corner and nothing else. After all in 2018, it is not beyond the wit of man to actually display a message that explains what the error is so at least you can start looking in the right dorection for a solution.
But fear not, there is an easy way to get your website back.
You will need to log into your hosting control panel and change the PHP vesion for 7.x to 5.6.
This will at least get your front page to load. You can then try and work out which of your components or plgins is causing the error at your own leasure and when sorted, change it back up to PHP 7.x. This will quite likely be resolved by updating your plugins and components.