Confirm what's actually broken first
'Checkout not working' covers several distinct failures that need different fixes: a blank or partially-rendered checkout page (usually a JavaScript error or a block/shortcode conflict), a checkout page that loads but won't accept payment (a gateway configuration or API issue), or checkout that appears to work but orders don't complete (often a webhook or redirect problem between WooCommerce and the payment processor). Reproduce the failure yourself with a real test transaction if your gateway supports test mode, and note the exact point it breaks -- that determines which half of the stack to investigate.
Check the browser console for JavaScript errors on the checkout page itself before touching any server-side settings. A single broken script from a marketing pixel, a page-builder widget, or an outdated payment-gateway extension can silently stop the checkout form's JavaScript from initializing, which looks identical to a server problem but has a completely different fix.
Isolate without breaking orders in flight
WooCommerce troubleshooting often recommends deactivating plugins one at a time to find a conflict -- reasonable for a low-traffic content site, risky for a store that might have customers mid-checkout right now. Where possible, reproduce the issue on a staging copy instead of live. If you must test on production, do it during the lowest-traffic window you have, and avoid deactivating anything that handles payment processing, tax calculation, or inventory directly, since disabling those mid-transaction can leave orders in an inconsistent state.
Check the WooCommerce > Status > Logs page before deactivating anything -- gateway API errors, webhook failures, and PHP fatals related to checkout are usually logged there with enough detail to skip straight to the actual cause instead of trial-and-error deactivation.
Common root causes and what they mean
A gateway API key that expired, was regenerated, or is pointed at the wrong environment (test vs. live) is one of the most common causes of 'checkout loads but payment fails' -- always confirm gateway credentials match the current live/sandbox mode before deeper debugging. A caching plugin serving a stale checkout page is another frequent cause of blank or broken checkout pages specifically, since checkout must never be served from cache; confirm your cache configuration excludes cart and checkout URLs.
Once the cause is identified, protect the fix the same way you'd protect any other production change: snapshot the affected plugin or configuration before changing it, apply the fix, then run a real test transaction to confirm checkout genuinely completes end-to-end -- not just that the page loads. WPFixAgent's WooCommerce-aware monitoring keeps plugin inventory and incident history together so a checkout failure can be compared against exactly what changed on the site around the time it started.