Sometimes after a Windows update, or after a QGIS update Windows users see the dreaded “Oops, looks like an error loading QGIS’… Could not load qgis_app.dll…” message
In short it means that one of the main libraries of QGIS cannot be fully loaded, because it is actually depending on other libraries, which (apparently) are not OK or available.
The 2 main reasons (I am aware of, please correct me if I am wrong), actually depend on your type of QGIS installation:
1) if you have installed QGIS with the “QGIS Standalone Installer” (the .msi version), the reason can be that the ‘opencl.dll’ version or install is messed up by Windows. For example see: https://answers.microsoft.com/en-us/windows/forum/all/opencldll-is-missing/de5a9687-c53d-4f33-8b28-47dc8115e745 As you can see it is not only QGIS having issues with it.
The solution is to make sure you have the good copy of opencl.dll either in the QGIS install dir OR in the c:/windows/system32 directory. See https://github.com/qgis/QGIS/issues/44806#issuecomment-908976764 for more info about it.
OpenCL is a framework to make it possible to share/move calculations to your graphics-card for parallel/faster processing: https://en.wikipedia.org/wiki/OpenCL.
(also check the second point below)
2) if you have installed QGIS using the OSGeo4W network Installer, the opencl.dll can also be a problem, BUT in that case there is sometimes an issue in which the scripts gis-bin.env or qgis-dev-bin.env files are vanished from your system. Those env files (in the QGIS/bin directory) are there to define the right PATH’s for QGIS and it’s libraries to find other libraries or elements needed.
As said sometimes (virus scanners?) do remove that script(s, one of each version of QGIS), so be sure those are there, or copy them from an other QGIS installation.
3) another opencl and QGIS related issue, see https://github.com/qgis/QGIS/issues/45507, is the one which tells you: “Can not find procedure entry point clCreateCommandQueueWithProperties in the DLL file C:\OSGeo4W\apps\qgis\bin\qgis_core.dll” or similar.
The solution to that is also to copy the right opencl.dll to make it available to QGIS again.
Hope this helps some people.