From time to time I've noticed that HTC One's keyboard doesn't appear or is very slow at appearing when you click in a text box to try and type. There is a simple fix for this thankfully...
- Go to Settings (pull down the menu from the top and tap the cog)
- Tap on "Apps"
- Swipe across to the right hand side so you can see all of the apps
- Tap on "HTC Sense Input"
- Tap on "Clear Data"
And that's it! You should be able to test this immediately by starting a new text message.