Cannot enter more than one letter/numeral per cell!

Hello all, I have been using excel 2003 with no problems. I have now got 2007 BETA. All worked well until about three weeks ago. Now I cannot enter more than a single letter or a single number in any cell! Plus, once entered I cannot delete!

I believe I may have inadvertantly altered a setting? Any advice would be gratefully appreciated! Thanks for reading this. Mike T

Hello,
Thank you for reading., In answer to your question: No, I get no messages or errors etc. I just fail to be able to enter anything. The curser remains after the letter and that is it! Mike T

Hello, Thank you for reading this. In answer to your question: No, it is not a shared notebook, in fact I am the only one using it. I believe I may have inadvertantly altered a setting? Mike T