Cell contents have all moved to the right of the cell

Hmm. Try this then:

1. Create a new Numbers document. The "styles" should be the defaults.
2. Select a header cell
3. Copy Style (this is found in the format menu, it is not the usual "copy" operation)
4. Go to your messed up document
5. Select a header cell
6. Paste Style (this is found in the format menu)
7. Use that Update button to update the style for all the other header cells
8. You will have to reapply centering to the text

The "update" button shows up if you change something about the "style" in the cell, like changing the font or font size. I've never used "styles" in all the years I've used Numbers but it appears "Table Style 1" is the default style applied to the header cells. If you hit the "update" button, it applies whatever changes you made to that cell to all the other cells that have that same style.

I'm trying to figure out what happened so you can correct it. The only thing I have come up with so far is the First and Left text indents got changed to something other than 0. It is in the Layout section in the Text tab.

I am now thinking it has nothing to do with paragraph styles and is some kind of bug/glitch.

I don't think numbers wrap in the cell (except when you are in the middle of typing one) but they are having the same problem as the header text where there is that dead zone on the left side of the cell.

If you add a new table to your document is it messed up too or is it normal? Assuming it is normal, here is something else you might try to correct the problem:

1. Select your entire table (click on the bullseye at the top left of the table)
2. Cut (Command X)
3. Paste and Match Style (found in the Edit menu) right back to the same spot.

Formulas in other tables that referenced the old table will reference cells in this new one. Charts made from the old table will now get their data from the new table. Hopefully this will get things back to square one and then you can reapply the format changes you actually want.

This is really screwed up. I'm running out of ideas. I feel like I'm missing something obvious but I don't know what.

Here are two more ideas, starting with an easy one.

1. Select a table
2. Go to the Table tab of the format sidebar and select a different table style for the table. The top left is the default one and is probably the one you are using so pick one of the others. Or maybe click on that one and see what happens.

Did that fix the problem? The formatting may not be what you want but this would be a good start if it fixed this weird problem. If it worked we can work on restoring the old table style (a correct version of it).

A more difficult solution but, if it works, it gets you a fresh document:

1. Select the entire table
2. Copy
3. Create a new document
4. Delete the default table in that new document
5. Paste and Match Style onto the blank sheet

Does it turn out correct (but with the default table formatting) or is it still messed up?

If that works, any tables that stand alone (do not reference other tables, are not referenced by other tables, and are not used for any charts) can be copied over to a new document this way. Any tables that do not stand alone (and related charts) will have to be cut/pasted into a single sheet in your old document with all the others they reference then you can select everything on the sheet (Command A), Copy, Paste and Match Style to a blank sheet in the new document. From there you can cut/paste things to different sheets to separate them again.

Whatever it was it appears to have affected anything with text in it, not just tables. Very very strange.

The only button to indicate a post is helpful is the "upvote" arrow. There is no button for "solved". These forums used to have buttons for "helpful" and "solved" that only the OP (original poster) could click but they took those away and replaced them with "upvote" (and I suppose the other is "downvote") which anyone can click. I assume the thinking is that there might be more than one solution and the best one will eventually get the most upvotes and float to the top. I liked the old way.