Cannot Access CF-Admin on one instance of CF 2016 Enterprise. update 13

Since you say things were resolved by a CF restart, I am willing to bet that the problem was some transient error, resolved by that restart. Common problems are outofmemory errors, as you may see reported in the minutes/hours before the outage, in the coldfusion-error.log. My money (given the report of "blank pages") is on that you will find you had outofmemory metaspace errors.

If so, there are many reasons it can happen, but the simplest solution is to remove the maxmetaspace arguments in the CF jvm arguments. That will allow it to grow beyond the artificial limit that Adobe sets (default of 192m), or that you may have set, or whatever may have been imported from previous CF installs (when the argument may have existed as maxpermsize, as per Java 7 and below).

I could elaborate on why you can just remove it (and what can cause the metaspace to rise and hit that artificial limit), and I plan to in a blog post. For now, if you had that error in the coldfusion-error.log, remove this argument (carefully). It would be safest to remove it via the jvm.config, in the cf cfusion/bin--or instancename/bin, since you say you are using instances. Save a copy of the file, and then edit it and make the change, saving the change (keeping it open) and restart CF. That way, if you may have made a mistake in the edit, you can easily "undo" it (even in lowly Notepad, if on Windows, since it's a single argument being removed).

Let us know if this is the problem (in your log), and if this resolution helps. (You can search the log to see if you had other metaspace errors in days and weeks before. If you did, this change would address all such previous hangups, though you may not notice it until you have had days/weeks of no more outages due to this.)