Assuming you haven't set the view mode to Perspective, this occasionally happens to me, in different ways. You'd have to upload a file and describe the problem in more detail for a specific fix. If that is untenable, then here are the generic fixes that I use:
First thing to try is to save, close and re-open the file.
If that doesn't work, try setting your view to World Plan and the other Orthogonal views in sequence and determining if it is only one object that cannot be seen orthogonally, or all of them.
If it's just one of them, then I expect that you accidentally rotated it and have to re-align it to your axes.
If it's all of them, you may have accidentally changed your reference viewpoint. I don't know how to fix that.
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