If I hold a letter, my keyboard stops working for that app
I cannot reproduce the issue on my end but it sounds like you may have an issue with an Accessibility feature, seeing as how it's happening in multiple applications. Have you tried toggling 'System 
Preferences > Accessibility > Keyboard > Sticky Keys' (or Slow Keys) on and off and trying to reproduce the issue?

Second option might just be that your keyboard settings were somehow changed from the preset delays that you're used to. You could check 'System Preferences > Keyboard > Keyboard' and check the delay sliders. Setting the first one to 'Fast' and the second one to 'Short' should actually allow you to press and hold a non-alpha-numeric key for multiple entries (such as the punctuation keys or keys like =, +, -, _, etc.) 

Note: The Mac keyboard does not allow you to press and hold letters because certain letters such as 'a' and 'e' trigger an accent menu to create characters such as 'à' or 'ē'.