When a store owner messages me "my product options disappeared," the first thing I do is resist the urge to start changing things. Missing variation dropdowns look dramatic, but the cause is usually small. What matters is the order you check in, because each step either fixes the problem or removes a whole category of causes.
I think about it in three layers.
Layer one is data. Is the product actually a variable product? Are the attributes marked for variations and saved? Does every variation have a price, an enabled checkbox and a sensible stock status? WooCommerce quietly hides a variation that has no price, which catches people more than anything else. This layer takes five minutes and solves a surprising share of cases.
Layer two is the environment around the product. Does the theme override the add-to-cart template, and is that override out of date? Did a plugin or a page builder template change what prints on the product page? A custom single-product template without the Add to Cart element will show a product with no options at all. I test with a default theme on staging to see which side the problem lives on.
Layer three is delivery. The dropdowns are JavaScript, so anything that delays, combines or caches scripts can break them. I check the browser console first, then purge every cache, then pause optimization settings. If that fixes it, I turn optimization back on with exclusions instead of leaving it off.
Two smaller things are worth knowing. Products with more than 30 variations are handled differently by default, and a filter lets you raise that limit when you really need to. And if nothing makes sense, turning on debug logging to a file often points straight at the cause.
I try not to touch live data before taking a backup, and I try to change one thing at a time. When a store is losing sales, it is tempting to try five things at once, but then you never learn which one worked.
I put the full checklist, with exact settings and snippets, here: https://amanurrahman.com/blog-post/woocommerce-variable-products-not-showing-fix
Do you have a fixed order you follow when you debug a store problem?