A blank white page, a critical error message, a 500 from the server, or a site that got slower and slower until it stopped. The error you can see is almost never where the problem is.
What it usually looks like
- A completely white page with nothing on it
- “There has been a critical error on this website”
- A 500, 502 or 503 from the server
- The front end works but wp-admin does not, or the other way round
- It breaks for a few minutes, comes back, then breaks again
What actually causes it
- A PHP fatal error after a plugin or theme update
- Your host bumping the PHP version under you, leaving old code that no longer runs
- Memory limit exhausted, usually by one badly written plugin
- PHP workers exhausted by bot traffic hammering URLs that do not exist
- A corrupted database table, often the options table
- The disk being full, which breaks things in ways that look like nothing else
What I do
Start with the actual error log rather than guessing, because a white screen is PHP suppressing a fatal error and the log has the file and line number in it. From there it is disabling plugins from the filesystem or over WP-CLI, checking the PHP version and memory limit against what the code expects, and repairing tables if it comes to that.
The intermittent version is different again. If it works for five minutes then dies, that points at resources rather than code, and the fix is on the server rather than in WordPress.
What it costs
Diagnosis is a fixed £150. You get the actual cause in writing, whether anything is recoverable, and what fixing it takes. The repair is quoted after that, once I know what I am dealing with. Nothing gets touched before there is a verified backup. More on how it works.