It's possible that you accidentally reassigned it. If this is the case, to
get it back, click the Home tab in the ribbon, and try this:
1. Press Ctrl+Alt+[plus sign on number pad]. This should turn the mouse
pointer into a cloverleaf pattern.
2. Click the Bold tool in the Basic Text section of the Home tab.
3. This should cause the Customize Keyboard dialog to appear, with the focus
in the Press new shortcut key: box. Press Ctrl+B.
4. Click Assign, then click Close.
Ctrl+B should now work again, unless something has gone haywire with your
keyboard (or unless you've accidentally toggled foreign keyboard support for
which Ctrl+B does not work).