Unable to access WP login

Hey guys
When I try to access my WP Login Panel I’m getting a message saying that:

The https://www.alisonpettyphotography.com page isn’t working

https://www.alisonpettyphotography.com is currently unable to handle this request.
HTTP ERROR 500

The site is however, still fully working and active on https://www.alisonpettyphotography.com via my laptop – although it doesn’t seem to be working via my phone. So I’m not sure if this has any relevance to the issue?

Really hoping you guys might know what’s going on here. I’ve searched the net and here high and low but still unable to fix it.
Many thanks in advance
Ally