It is rather difficult to calculate the reputation exactly from [SEDE](https://meta.mathoverflow.net/tags/data-explorer/info)<sup>1</sup>, but the total score (i.e., the sum of the scores of all user's posts) can serve as a reasonable proxy.

Here is a query which looks at such users: [Deleted accounts above the given score and number of posts](https://data.stackexchange.com/mathoverflow/query/1470017/deleted-accounts-above-the-given-score-and-number-of-posts?minscore=500&minposts=50).<sup>2</sup> Notice that you can change the parameters minscore and minposts - the query returns users who have both score *and* number of posts $\ge$ the given values. For example, you can set on of them, [or even both, to zero](https://data.stackexchange.com/mathoverflow/query/1470017/deleted-accounts-above-the-given-score-and-number-of-posts?minscore=0&minposts=0) - to remove the restriction. (As you have already mentioned, once you have userid, you can find more about that user, such as the list of their posts: [Is there a way to check all questions/answers posted from a deleted account](https://meta.mathoverflow.net/q/3748). I will add that sometimes you might be able to find also a username that the user had before the deletion - the methods mentioned here work also for deleted users: [Recognizing identifiable users changing user names](https://meta.mathoverflow.net/q/4426).)

You can make a similar query which lists both existing and deleted users: [Users above the given score and number of posts](https://data.stackexchange.com/mathoverflow/query/1470018/users-above-the-given-score-and-number-of-posts?minscore=500&minposts=50). Of course, if we're making a query combining both deleted and existing users, we can include also some data from the table Users for the ones which are not deleted, like [in this query](https://data.stackexchange.com/mathoverflow/query/1470030/users-above-the-given-score-and-number-of-posts).

I can also imagine, that somebody might be interested in finding active user in some specific area rather than on MO in general. This query might possibly help with that: [Deleted accounts above the given score and number of posts in a specific tag](https://data.stackexchange.com/mathoverflow/query/1470034/deleted-accounts-above-the-given-score-and-number-of-posts-in-a-specific-tag).

<hr>

Re: *I have no idea how to find the userid of someone I used to know but have disappeared*. I will also add that if you have a post or a comment from a deleted user, you can see the userid there. You will find `userxyz` - where `xyz` is the userid - in places where you see username for the accounts which are not deleted.

If you remember the username and you want the userid, you might simply try to search [where the username was mentioned on MO](https://www.google.com/search?q=quid+site:mathoverflow.net) - which might lead you to some of their posts or comments. Or you can search for [comments mentioning this username](https://meta.mathoverflow.net/q/3247) - and you might be lucky enough that you'll find a comment responding to a comment from the delete user. Examples of such queries are [here](https://data.stackexchange.com/mathoverflow/query/556789/comments-containing-given-keyword-with-text-and-author?Word=quid) and [here](https://data.stackexchange.com/mathoverflow/query/556789/comments-containing-given-keyword-with-text-and-author?Word=@quid). 

<hr>

<sup>1</sup>Some discussions related to calculating reputation from SEDE: [How to retrieve exact reputation on "legendary day"?](https://meta.stackexchange.com/q/300600), [Reputation History from Data Explorer](https://meta.stackexchange.com/q/144716), [A query, days hitting rep cap vs days a member (rep)](https://meta.stackexchange.com/q/234720).

<sup>2</sup>Maybe you noticed the apparent discrepancy that for one of the users the column "Posts" shows a bigger number than the number of answers and questions together. The reason is that the table Posts in the database also includes the tag-wikis that the user created. (The details about this table can be found in the [Database schema documentation for the public data dump and SEDE](https://meta.stackexchange.com/q/2677).) Here is a query [which also includes tag-wikis](https://data.stackexchange.com/mathoverflow/query/1470035/deleted-accounts-above-the-given-score-and-number-of-posts).