My guess is you turned off the sidebar or maybe you did actually delete the code.
So here’s my best idea of the moment…
Try looking to see if you can reset something called ‘layout’ or something like that. You want the two column mode or layout.
Else…
Switch your theme to some other theme and then reinstall the theme you have. I think it’s called ‘delicious’ or something like that.
You can manually install the old theme by reloading the files from the installation zip… if necessary you can delete the theme and reinstall it from the themes dashboard.