This problem is already reported as bug in issue SNAP-671. I observed it on my computer (windows 7) too.
I did the following.
- loading a Sentinel-2 product
- resampling to 20 m spatial resolution a window opens giving the message: "The target product has successfully been created ... . Actual processing of source to target data will be performed only on demand ... .
- Within the box opened there is a button to click for "Don't show this message anymore"
- I clicked on this button and after some more actions using the mouse I recognized that there is no more keyboard input possible.
I found the way back:
I opened menu tools | options. The options window opens and directly on the first page is a point "other settings" with a button to click for "Show suppressed message dialogs again". After activating this option by clicking on the button I had my keyboard back.
I hope this helps the developers to locate the error and helps other users to get their keyboard reactivated.