When ArcGIS applications fail, or are behaving strangely, one of the first things to try is to reset your application template document (ArcMap's is "Normal.mxt") or the entire ArcGIS application profile. This resolves many problems with ArcGIS software. (This document only applies to ArcGIS Desktop (ArcMap, ArcCatalog, etc) not Pro.)
It's a good idea to do this as a every few months, or whenever a service pack or update is applied to ArcGIS, as these template files can gradually fill with cruft and slow down your ArcGIS Desktop applications.
These steps do not require elevated privileges / admin access, as all users own their personal %APPDATA% folder.
This may include killing any lurking "headless" ArcMap.exe, ArcCatalog.exe processes using the Task Manager.
2. Start Windows Explorer and open your application profile folder.
The location of the user profile depends on the operating system. This folder is normally hidden.
XP/2003/2000: C:\Documents and Settings\<your user name>\Application Data\ESRI
Vista/7/2008/8/2012: C:\Users\<your user name>\AppData\Roaming\ESRI
The easiest way to get there is to simply type the environment variable %APPDATA% (with the percents) into the Windows Explorer path box, which will take you directly to the folder that contains the application profile folder ESRI, regardless of Windows version, and whether hidden files and folders are turned on or not.
3. Browse within the ESRI folder for the application that has the issue
Each component of the software has a template that stores settings and customizations for that part of ArcGIS. These are:
4. Rename the template file (for example, Normal.mxt -> Normal_old.mxt).
When that component of ArcGIS is started again, a new, fresh template is created. Note that some your customizations (buttons, file connections) may have to be re-applied. This may be all you need to do.
5. Doing a complete application profile reset
Sometimes it may be advisable to entirely reset the entire user profile:
Close all ArcGIS applications
Rename the %APPDATA%\ESRI folder to another name, for example, ESRI-old
Some files and folders may be copied over from ESRI-old back to the ESRI folder. Key candidates to copy back are items are personal .style files (ArcMap folder), ArcCatalog.gx (ArcCatalog folder - contains folder connections), and toolboxes (ArcToolbox\My Toolboxes).