This trivial problem can be a big headache for many users.
Fastest solution:
For Windows XP, Windows 7
Press simultaneously Ctrl and Shift keys. Everything should go back to normal.
For Windows 8, 8.1, 10
Press simultaneously Ctrl and left ALT keys.
What happened there?
Ctrl+Shift or Ctrl+Alt keys combined represent default shortcut in Microsoft Windows systems switching input language. If you’d like to get rif of it for good go to:
Control panel ->Clock, Language, and Region -> Language -> Advanced settings -> Change language bar hot keys
Click Change Key Sequence and then select NOT assigned. Confirm OK to bring the change into effect.