Ctrl + Shift + Arrow stopped working

Check if you have not accidently activated the Scroll Lock button on your keyboard.

In case this has happened, Excel actually shows this in the status bar:

Then to activate it turn off scroll lock the button looks like the below link to google image search: Googled search of scroll lock

If you cannot find the button (e.g. on many laptops), you can use Window's virtual keyboard:

1. Just Search "On-screen Keyboard" in Windows Search.
2. Open Virtual Keyboard.
3. Disable the Scroll Lock key by clicking on it

Check if Transition Navigation Keys under "Lotus compatibility" is checked.

File -> Options -> Advanced, and scroll down to the bottom

That should be unchecked for the default behavior you had before

I've had this problem as well, easiest fix for me, save file, exit file, re-enter file, and it's fixed.