Hello @Aristeidis Dedes ,
Welcome to Microsoft Q&A forum.
Please check if following settings has been selected. If “Enable Web Forms designer (requires restart of Visual Studio)” option(checkbox) is unchecked, please select this option and restart your Visual Studio.
Tools > Options… > Web Forms Designer > General > “Enable Web Forms designer (requires restart of Visual Studio)”
If this issue remains, please let me know if you performed some operations before, such as updating Visual Studio, updating Windows System, installing some third-party applications, changing some settings in VS and other team members changed some configurations(maybe)...
Also, are you trying to open a plain HTML file, or a ASPX, a ASCX and a Master file?
Besides, if you create a new project does this issue remain?
Best Regards,
Tianyu
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