Formula computes with the value of zero.

I have developed a few simple spread sheets, where one spreadsheet is a summary of values computed on different spreadsheets, and everything was working well. Now, a reference value comes back as zero on certain cells, even when there is real number referenced from the other spread sheet.
Then at the same time, certain cells with formulas behind them compute to the value of zero, even with referenced cells from the same spread sheet.
Any ideas what has happened here?

You might have one or more circular references. Look at the bottom left. If you do, it should say so, and you can click on the 'Circular reference' notification to find the address of the most recent circular reference.

Circular reference it was!
It was obvious once I saw it.
Thank you

You're very welcome. If you are inclined, you can reward with an add to my reputation.