Make sure that all the layers you want to keep are visible. Choose Layer > Flatten Image, or choose Flatten Image from the Layers panel menu.

Can you adjust a layer individually?

You can edit only one layer in Photoshop using a clipping mask. With an adjustment layer placed above another layer in the Layers Panel, right-click on the adjustment and press Create Clipping Mask to clip it to the underlying layer.

What happens when I flatten an image with hidden layers?

If you have any hidden layers, Elements asks you whether you want to discard those hidden layers. Choose Flatten Image from the Layers panel menu or the Layer menu. All your layers are combined into a single background. If you mistakenly flatten your image, choose Edit→Undo or use your Undo History panel.