What the heck have I done to my Excel sheet (and how to undo it)?

Go to Review > Changes > Protect Workbook, and uncheck Protect Structure and Windows option. If you have set a password for it, Excel will prompt you to enter it. Otherwise, just hit enter to remove the structure and window protection.

On Excel 2011 for Mac, the process is only slighty different. On the Review tab, under Protection, click Workbook to remove the protection. See also the official online help.

One way that can be done is through VBA. See here: http://support.microsoft.com/kb/137033

And another VBA method: http://www.mrexcel.com/forum/showthread.php?t=41720

See if there are any macros associated with this sheet.

It's also possible that this is due to Workbook protection: http://webcache.googleusercontent.com/search?q=cache:02brqfAzeN4J:office.microsoft.com/en-us/excel/HP052035251033.aspx+remove+maximize+minimize+spreadsheet+window+excel&cd=3&hl=en&ct=clnk&gl=us&client=firefox-a

UPDATE: I've been able to duplicate that issue in Excel 2007, and it is a worksheet protection issue.

In Excel 2007, select the Review tab, then click the Protect Workbook button and uncheck Protect Structures and Windows.