user32.dll
This file is not a system component: windows itself, not a download. Restore it from the source that installed it rather than downloading a replacement.
If the file is already there and Windows still complains, read the message again: “entry point not found” is a different problem from a missing file, and installing the runtime again will not solve it.
And if the box says RUNDLL rather than naming a program, the fix runs the other way: something left an instruction behind pointing at a file that is meant to be gone, so restoring the file is the wrong move.
Do not download this DLL on its own
Searching for a missing DLL by name leads to sites that hand you that single file. It is one of the oldest and most effective malware delivery methods there is, and a system DLL from an unknown source is indistinguishable from a deliberately modified one. There is never a legitimate reason to obtain a runtime component that way.
The same goes for the tools that offer to do it for you: what a DLL repair tool can and cannot actually fix.
What it is
The Windows USER library: windows, menus, the message queue, mouse and keyboard input. Together with kernel32 and gdi32 it is one of the three pieces every graphical Windows program is built on. It has shipped with every version of Windows and no publisher distributes it.
Where you see it: Startup errors of the form "The code execution cannot proceed because user32.dll was not found", and in crash reports naming it as the faulting module.
The quickest way to see that the message is misleading: if user32.dll were really missing, you would have no desktop, no taskbar and no dialog box to read the error in. The window that shows you the error is itself drawn using user32. So the file is there, and something else is wrong. The usual cause is architecture: a 32-bit program must load the copy in SysWOW64 and a 64-bit program the copy in System32, and a program that ships its own stray copy next to its executable, or a launcher pointing at the wrong path, produces exactly this message. Second most common is that the DLL genuinely failing is one further down the chain and the loader reported the first name it could. If the system file itself is damaged that is Windows damage, not a download: run "sfc /scannow" and then "DISM /Online /Cleanup-Image /RestoreHealth" from an elevated terminal. Copying a user32.dll from a download site into System32 does not repair Windows, it replaces a file Windows owns with one of unknown origin, and Windows Resource Protection will fight you over it.
Why there is nothing to download here
There is no verified download we can offer for this one, because installing a file is not the fix. The steps above are what actually resolves it. That distinction matters: the sites ranking for this file name are selling a download that would not have helped even if it were safe.