Sometimes, major Windows updates can reset font priorities. If Zawgyi stops working, simply re-install the .ttf file.
If you are not comfortable manually installing files, you can download a "Zawgyi Keyboard and Font Installer" (often found on sites like myanmaros.com or similar tech blogs). These ".exe" installers automatically install the font and the keyboard layout. zawgyi one font for windows 10 64 bit free download
Once you have downloaded the .zip or .ttf file, follow these steps to install it on your 64-bit system: Sometimes, major Windows updates can reset font priorities