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.