I'm afraid we really can't offer much -- You've given no indication
whatsoever of what version of Excel you're using, what it's update level is,
or even what Operating System you're running it on. Even if you're using a
Mac there are at least 3 "current" incarnations of OS X, each of which has
its own set of keyboard controls.
Without that information we're as much in the dark as you are as to what the
problem might be... Or if it's a problem at all -- it may be normal
behavior, although Overtype mode isn't available in Mac Excel. If you are
using a Mac you need to provide all that information.
If you're using a PC version of the program you've simply pressed the Insert
key [sometimes labeled INS] in the Navigation Keys group. Press it again to
toggle Overtype OFF.
Regards |:>)
Bob Jones
[MVP] Office:Mac