What version of InDesign and what operating system, exact versions please 
slight edit - when was the last time you rebooted the computer, sometimes a reboot clears temporary issues.
Any plugins? Any 3rd party software, like VPNs, firewalls, Anti-Virus running? Do you use WebRoot?
Any new fonts recently installed?
A quick way to check is to start your computer in Safe Mode, open only InDesign and if it’s behaving normally you know it’s an external factor - you can then reboot in normal mode and then try eliminate the external factor.
As @Just-B says, resetting preferences and clearing caches are good steps.
Close InDesign. Relaunch while holding Ctrl+Alt+Shift (Windows) or Cmd+Ctrl+Option+Shift (Mac) and confirm deletion of preferences.
Instrusctions below and a nuclear option.
Manually delete the InDesign cache and recovery folders and restart InDesign
Windows
Delete the contents of:
%LOCALAPPDATA%\Adobe\InDesign\Version 21.0\
Mac
Go to:
~/Library/Caches/Adobe InDesign/
and
~/Library/Preferences/Adobe InDesign/
Delete the relevant InDesign version folders.
Remove Recovery Data
If documents are crashing or opening strangely, also delete the contents of:
Windows
%LOCALAPPDATA%\Adobe\InDesign\Version 21.0\en_US\Caches\InDesign Recovery
Mac
~/Library/Caches/Adobe InDesign/Version 21.0/InDesign Recovery
Nuclear option
https://helpx.adobe.com/ie/creative-cloud/apps/troubleshoot/diagnostics-repair-tools/run-creative-cloud-cleaner-tool.html