Many people complain about missing dll files all of a sudden in windows.
Dll files are runtime files. That is, when a software runs, it requests the dll file during its runtime. If the software cannot find the dll file or if it finds the dll file with an unrecognized version then they throw a dll file error message.
Some times new programs when installed overwrite the older dll files with newer versions. When the older program requests for the dll file and if it is not able to find its older version, then dll file errors occur.
These missing dll files maybe the out come of some other softwares , which during their uninstallation remove some dll files along with the other files to be removed.
A bad sector in the hard disk at the location of the dll file can also result in errors.
Also, the simple reason is the accidental deletion of the dll files manually by the users.