Intermittently cannot access site or admin

Over the last few days, when I’ve tried to access the WP admin pages of my sites, I’m greeted with “HTTP ERROR 500- the site cannot currently handle this request” (see screengrab).

The only way I have been able to access it indirectly via logging into the site itself (as if i were a site user), and then clicking ‘Edit Page’ on a page and getting into the admin that way. If I try to access it using the black bar at the top when logged in as a user I get the same HTTP error message.

This morning, having logged in via that method and updated all plugins on the site, I am unable to access the site or the WP admin and get the same message even when just trying to view the site directly (www.liverylist.co.uk). I get the same error message on my PC and if I try on a mobile device the screen is just blank.

In addition, the ‘Go to Checkout’ button has been redirecting people to the homepage since yesterday.

I installed Woo Commerce last week but did not complete fully setting it up and was going to go back to it (this did not seem to make any difference to the access issue or checkout button issue as was all still working via the original PayPal set up when I tested it). When I realised the checkout button issue this morning, and having decided against WooCommerce, i deactivated all related plugins this morning, and ensured all setting were as they were previously 5WP invoice via PayPal payment gateway). People have managed to make payment without issue over the last few days via the previous PayPal system but I notice from unpaid listings that the checkout button has only been an issue since last night. When I was testing this morning, the checkout button did nothing until it had been clicked several times and then redirects to the homepage.

Unsure if the issues are somehow linked? I cannot see if updating plugins and removing woocommerce has solved the checkout problem as i now cannot access the site either as admin nor user.

Any suggestions as at the moment the site is down and I’ve people uploaded listings that are unable to pay?

Thanks