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Can you provide more info, like OS version and what version of Photoshop you have.
Are you getting an error message, what are your PC specs.
When saving images for the web is going crash Photoshop
What have you done for trouble-shooting so far?
Restoring Preferences after making sure all customized presets like Actions, Patterns, Brushes etc. have been saved might be a good starting point:
Which format are you trying to save, what are the other settings, what are the image’s pixel dimensions, …?
Have you tried resetting the Preferences?
In particular ~/Library/Preferences/Adobe Photoshop CC 2018 Settings/Save for Web Prefs.psp