If the standard toolbar is visible but all of the buttons are greyed out, run Excel's Detect and Repair. Detect and Repair will automatically check for problems in Excel such as missing files or bad registry settings. If it finds any problems, including those that might be causing your standard toolbar to be greyed out, it will automatically repair them.
To run Detect and Repair, go to the Help menu and click on Detect and Repair. The Detect and Repair dialog box will open. There are two

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optional choices on this dialog, which are Restore My Shortcuts While Repairing and Discard my Customized Settings and Restore Default Settings. If you have created custom shortcuts in Excel and want to keep them, make sure to select
Restore My Shortcuts While Repairing. If you do not, Detect and Repair will remove those shortcuts.
Selecting the Discard my Customized Settings and Restore Default Settings option will restore several settings to default, such as the Office Assistant, recent entries on the File menu, and menu or toolbar customizations.
Once you have made your selections, click Start and wait for Detect and Repair to find problems and hopefully fix your greyed out standard toolbar.