If you open your 3dsmax and found the UI has been reset or change to other UI, it could be due to:
- the CUI files are corrupted due to unknown reason
- last 3dsmax session was not shutdown properly
- your CUI files are located on the network and you have lost access to it
To reset your UI to default configuration, simply go to Customize > Custom UI and Defaults Switcher...
Then select your preferred UI and restart 3dsmax.