Had the same issue, some how I switched to another viewport.Which did not contain the axis, in the left bottom corner of the viewport.
I got it back by, switching to another viewport.
Click on the icon of the “two overlapping monitors” in the right top screen of the viewport.
Once you click on that, depending on your viewport settings, you should see 4 viewports now.
Some might be black, choose whatever viewport and change the camera to perspective, now you will see the axis in that viewport again.
Next maximize this viewport by clicking the top right monitor icon in your viewport.
Now it’s all good for me again!