My object has been rotated randomly for some reason on all axis. When I import it into Unity the rotation still applies. How can I reset it so it's the right way up?
The rotation which was done in Edit mode can't be reset as it would be possible with rotation in Object mode. Depending on the case, it's possible to use workaround.
Grab empty, hovering the mouse over faces which should be rotated along world axes. I.e. in case of building this will be faces representing the floor.
Once empty is snapped, it gained new rotation values. Parent object to the empty with Ctrl+P.
Select empty and clear its rotation transforms (Alt+R). Object will follow changed rotation and should become aligned along world axes (or at least roughly aligned).
Clear Parent with (Alt+P) by selecting Clear and Keep Transformation.
Select empty, then delete with X.