just change .centerColumn back to float:left;
But the above doesn't explain why your sideboxes aren't working. Are both column left and right set to 1? How many sideboxes do you have turned on in "Tools" "Layout Boxes Controller" (remember after changing templates you have to turn any sideboxes you had on back on). Have you made any changes to common/tpl_main_page.php? Maybe try re-uploading that one.
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