My Kitchenaid Toaster only toasts on one side. The bagel button is not turned on. The element inside does not heat up. Can this be fixed?

There are tow issues here: Bilateral elements (heating strips) of course are necessary for two sided toasting, and you mentioned the Bagel Setting. Are you saying the Bagel setting works with both sides in operation and inoperative when in normal mode? Obviously is in both modes, should only one side function, the element is burnt out.
We need more information.

Because the nichrome wire on one side of the toast has broken.
Its time to get a new toaster.

You definitely don't have the "bagel" button pressed, right? If it is in bagel mode, it only uses the heating elements on one side.

I have this toaster. You have to turn the dial up to 6 or more and the lights light up

Each bay has 3 buttons: Left button: Frozen items (Waffles, etc.)
Center button: Bagel setting ( turns off the outside elements, toasting only the insides of the bagel)
Right button: Cancel (turns off the Frozen and/or the Bagel settings and "pops up" the toast.

Hola tienes que desarmarlo quiz?s se movi? de d?nde ten?a que estar y por eso no funciona

I think it is set to cook bagels. Check the control settings.
Most toasters have a bagel setting which only cooks the
sliced side of the bagel. This model is designed to accept bagels. Hope this helps.

You either have a stuck/defective bagel button, which turns off two elements; the elements have a break in them; or there is a wire broke or loose on the inside elements.

If you can use a multimeter, you can unplug the toaster and remove the case and check for all of these conditions. At each end of the toasting element, check for continuity, if OK, then check the bagel button to see if it is working correctly, again with the continuity check.

Check all connections to make sure none have burned off at the wire/connector points, and finally, check the thermal fuse to see if it might have failed.

Other than that, there isn't much else to check.

Hope you found this Very Helpful - Best regards!

Please ensure that the "Bagel" function is not selected (LED is not lit up). The Bagel function will toast one side of the bread while only warming the other side.