One way is to reset all the toolbars to factory defaults. This will mean that
you'll lose any customized changes to any of your toolbars.
Close excel
windows start button|search
look for *.xlb
rename all of them to *.xlbOLD
Restart excel and see if that fixed it.
If it did, you can delete those *.xlbOLD files.
If it didn't help, just close excel and rename them back to *.xlb.
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You could also be a little more selective.
Open excel
hit alt-f11 (to get to the VBE where macros live)
hit ctrl-g (to see the immediate window)
type this and hit enter:
application.commandbars("worksheet menu bar").enabled = true
Then back to excel and try to use:
tools|customize to reset/show any toolbar you need.