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 🙂
[Edited by Rickmon, 1/1/2020 1:02:56 AM]
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.
Install .NET CORE v3.1 from Microsoft.