Scroll bar

Further to all other advice received so far from @Bill Silbert  and @Robert at ID-Tasker 

Try resetting your preferences:

Windows: Start InDesign, and then press Shift+Ctrl+Alt. Click Yes when asked if you want to delete preference files.
macOS: While pressing Shift+Option+Command+Control, start InDesign. Click Yes when asked if you want to delete preference files.
A dialog will appear asking to delete the InDesign Preferences, select 'Yes' on this dialog.
Further info and instructions here if needed:
https://helpx.adobe.com/in/indesign/kb/indesign-preferences-support-file-locations.html

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Reinstalling rarely fixes everything - you can do a complete reinstall using the
Creative Cloud Cleaner Tool https://helpx.adobe.com/creative-cloud/kb/cc-cleaner-tool-installation-problems.html

Are you sure that you haven't simply expanded the document window so that the scroll bar is actually below the bottom of the screen? Try pulling up on the window from the top of the window and see if the missing scroll bar is now showing. If so then grab the bottom right corner and pull it upward to keep the scroll bar in the viewing area.

Unfortunately, we are not mid readers. 

What platform, full OS version, full InDesign version? 

Can you post a screenshot? 

Try resetting preferences:

https://community.adobe.com/t5/indesign-discussions/reset-indesign-preferences-and-other-troubleshooting/m-p/11990234