Can't maximize spreadsheet

Got a nice wide monitor so I can fit lots of cells on screen, but one of my spreadsheets just sits there occupying the left half only.
Last week, I ran two sheets side by side to copy / paste a bunch of text. It didn't even involve this sheet, but now I can't change this sheet to use the whole screen. Can't find the solution in my Que book. I'm hoping this forum works better.
What am I doing wrong?