Reinstall Word

Try these steps in order:

1. Check whether Word is stuck in Immersive Reader.
   • If there is an Immersive Reader tab on the ribbon, open it and select Close Immersive Reader.
   • This can make text appear abnormal and prevent normal editing behavior.
2. Turn off Word add-ins.
   • Go to File > Options > Add-Ins.
   • In the Add-ins list, note the Type for an add-in.
   • In Manage, select that add-in type, then select Go.
   • Clear the check boxes for the add-ins to disable them, then select OK.
   • Repeat for other add-in types.
   • Test whether font changes work again.
3. Check the Normal.dotm template permissions.
   • Open File Explorer.
   • In the search box, type Normal.dotm and run the search.
   • Right-click Normal.dotm > Properties.
   • On the General tab, make sure Read-only is not selected.
   • On the Security tab, select the user name and confirm Write permission is allowed.
   • If these settings cannot be changed, contact the company IT administrator.
4. Repair Office.
   • Close all Office apps.
   • Open Add or Remove Programs from Windows.
   • Select Microsoft Office or Microsoft Office Word from the installed programs list.
   • Select Change.
   • Select Repair or Repair Word (Repair Office), then continue.
5. Reinstall Office if repair does not fix it.
   • Reinstall is the more aggressive option and resets Word to default settings except settings stored in Normal.dot or Normal.dotm.

If the problem is only that the default font will not stay changed, set it again in Word:

• Go to Home and open the Font dialog box launcher, or press CTRL+D.
• Choose the font and size.
• Select Set As Default.
• Choose All documents based on the Normal template.
• Select OK twice.

References:

• The font on all Word documents has been changed and I cannot change it back. - Microsoft Q&A
• Change the default font in Word
• How to reset user options and registry settings in Word