Just tested the Black Mesa trainer...it is working great 😍
CCleaner might help, also clean/empty your browser's cache, run as administrator...re-start your PC, ... follow the directions here: Link.
Hope it help 🙂
Try a reboot and see if it runs again. If not then something must be blocking it. Likely AV, firewall or other software such as RivaTuner or MSI Afterburner. Also make sure both .NET cores are installed. They may be damaged.
Have both these .NET CORE and FRAMEWORK Installed from Microsoft.
.NET Core 3.1 Desktop Runtime x86 Link
.NET Core 3.1 Desktop Runtime x64 Link
.NET Core Current Runtime x86 Link
.NET Core Current Runtime x64 Link
.NET Framework (get latest) Link
(Both x86 and x64 of the .Net Core MUST be installed)
Reboot PC when done.