I suddenly started to get the following error message: 'your system does not use 3D hardware accelation. Blender requires a graphic driver with OpenGL 2.1 support'. So I searched the internet and installed opengl32.dll. Blender is now opening again, but I'm hardly able to manipulate in the viewport. Blender is very slow to respond whenever I zoom or rotate in the viewport. Does anybody know a solution?
This really sounds like a driver issue. Maybe something got corrupted or deleted (it can happen that an antivirus, for example, deletes a critical component of a driver). You see, OpenGL usually is part of the GFX driver package for any graphics card because OpenGL is hardware-dependent and is integrated by the GFX card's maker. Just putting the file in there is not a solution--you're doing the equivalent of driving a car with a wheel missing.
For this reason, I recommend that you reinstall the current version or install the newest version of your graphics drivers. If you already have the newest, either try reinstalling or roll back to an older version if that is the cause (most likely it's not).
If you have an AMD card, download drivers here: https://www.amd.com/en/support
If you have an NVIDIA card, download drivers here: https://www.nvidia.com/Download/index.aspx
If this doesn't work, you may need to uninstall the drivers first (to avoid left-over files from conflicting with the new ones). Otherwise, an update should suffice.