Newly entered data disappears

Hi I am a new member and am hoping someone can help me...I have muddled through with an Excel template set up for me by my accountant years ago to do my annual accounts spreadsheets for my tax return (due in 8 days!). I normally create new sheets by using the Move & Copy command and wiping the contents with no problems, and the formulas move too. This time, every time I enter data in the fresh sheet it just disappears whether I press Return or just click another cell. The figures don't show in the Total bar either even though the formula seems to be showing ok in the formula bar.
Even more odd, when I tried clearing contents from an existing sheet and enter new data (without moving or copying anything) the same thing happened. Nothing in the cells!
I have the text colour set to black by the way and format to Number. As you can tell I am a complete beginner so simple answers appreciated!

Attach a sample workbook or put a link to it here
Make sure there is enough data to demonstrate your needs
Include a BEFORE and AFTER sheet in the workbook to show what you're trying to achieve
Make sure your desired results are shown, mock them up manually if necessary
Remove any sensitive data

Thanks Special K I have created a new 'dummy' worksheet from an existing one exactly as I do normally when I make a new workbook. I then made a new worksheet for the next month using Move & Copy Sheet and the data entered fine on the first lines in the new sheet. I then erased the old data and replaced with new and the problem started again. No data shows in Totals, the formula is not working (as the cells are 'empty') and any data I enter into the cells in the Totals column just disappears and the cell remains 'empty'. It is doing the same in existing workbooks so is it a setting somewhere maybe? I know this is very basic and I really do need beginners lessons!

ps I have just discovered that if I use a comma instead of full stop for the decimal point the entries are entering and Totals formula works as well as the link to the breakdown on the right page. Is this all that's caused my problem?

I think after hours and hours of searching, I have found the source of my problem. The Language and Region settings on my AppleMac were set to France. They use a comma instead of full stop as the decimal point. I changed the setting to UK and my data is entering as normal and formulas all work. It appears to be a common problem now I have found it, for users working with International clients, so perhaps it will be helpful for others.

ps Once you set your system preferences for the currency style, you must quit and restart Excel for the selection to take effect.