Need help with formulas that don't calculate

I've attached an example sheet that I've setup to keep track of Fuel and Cooking cost's. In setting up the formulas I ran into a situation I've never had before and couldn't find any
reason for. I tried many different things and finally just gave up and come for help.
In the sheet provided you'll see several boxes that are colored in red with gray letters, these are the cell's I've been trying to get formulas to work in. For and example of what I've been seeing if I even used the simple SUM feature to add the cells in a column it excel wouldn't even provide the answer when tested. I'm at a loss.

1st Thank you for the reply. However I had already tried to use that simple formula but it didn't provide nothing more that zero. Regardless I did retry it again and without the <>"" and still received the same result of $0.00 I was going to provide you a screen shot of the page showing the formula and the result however I couldn't figure out how to do it so I'm typing instead. Still don't understand why the cells won't allow calculation when the rest do.

You have circular references which is why you are getting a result of 0.
The formula in col Q is looking at col P & col P is looking at Q

OMG !!!!!!!!!!!!
That was the problem thank you ever so much !

One question though how were you able to find the problem so I may find one the next time ?

I got the circular reference error warning as soon as I opened the workbook, when I then looked at the left hand side of the status bar it mention Q41

Thank you I'll try to remember to look for that in the future.

Glad to help & thanks for the feedback.