Quick answer
The short version
Search history with words that were on the page rather than the site's name, narrow to the period you think you saw it, and check your screenshots, the clue is often an image you forgot taking.
01 · Method 01
Search browser history by remembered terms.
History search matches titles and URLs, so search topics: a product category, a technique, a city, a word that would plausibly appear in the page title. One useful word can be enough.
02 · Method 02
Search bookmarks.
If you saved it, search broad subject terms rather than the site name. You bookmarked it because of what it was about; the name is the part you never rehearsed.
03 · Method 03
Use approximate dates.
Narrow history to when you think you encountered it. Even a month, or “during that project”, converts an unbounded scroll into a readable list.
04 · Method 04
Search distinctive phrases.
If you remember a phrase from the page, search it directly, in your history, and in a web search engine in quotes. Unusual phrasing is the single most identifying thing a site gives you.
05 · Method 05
Look for screenshots.
People screenshot a site without consciously remembering that they did. Your screenshot library may hold the answer: the logo, the URL in the address bar, the layout you're trying to describe.
If your screenshots are searchable by their text, search the phrase there too.
06 · Method 06
Use the surrounding project or topic.
Where were you when you found it? What were you researching? Which other sites did you save that day? Sites arrive in sessions, and a session is a much smaller haystack than a year.
07 · Method 07
Use visual memory.
If your tools support visual retrieval, describe it: monochrome, giant typography, a portfolio, a specific kind of grid. This is frequently the strongest memory you have, and the last one people think to use.
What you remember
“That monochrome portfolio with giant typography I saw while researching design studios last summer.”
What you get back
A short list of sites from one research session, narrow enough to recognise on sight.
08 · Where Shelf fits
When you kept it, this is easier
Anything you kept in Shelf, the page itself, or a screenshot of it, carries its text, a description, its date and its neighbours. So “monochrome portfolio, studio research, last summer” is a query rather than a shrug.
For pages you never saved, the methods above are still the right order. Shelf only helps with the things you chose to keep.