When I don’t take the time to use a tripod, my horizons are often not straight.  Here’s an example:

Crooked Horizon

To fix this in Lightroom, in the Develop Module tool drawer underneath the histogram, click on the Crop Overlay tool (R).    Crop options appear beneath it, and a grid is placed over your image.

Crop Overlay Tool

Now you have two choices.  The first is to rotate the image visually.  Hover the mouse outside of a corner of the image until the double arrow appears.

Rotating visually

Click and drag the cursor up or down, aligning the grid with your horizon.

Straightened Image

When you are satisfied, hit Enter/Return or click again on the crop overlay tool symbol to exit crop mode.

The second method is to click on the ruler (i.e. the Straighten tool) in the crop options box to select it, and then click and drag it over your crooked horizon — the whole horizon or just a part of it.   Lightroom will calculate how far off from level the horizon is, and rotate the image the proper amount to fix it.

The Straighten Tool

Drawing Along the Horizon with the Straighten Tool

When you are satisfied, hit Enter/Return or click again on the crop overlay tool symbol to exit crop mode.

With either method  you do lose part of the image — this is the price you pay for not getting it straight in camera.

You can use the same methods in Camera Raw.  The Straighten tool is right next to the Crop tool.