How Do You Make A Formula Stop At A Blank Cell?

HOW DO YOU MAKE A FORMULA STOP AT A BLANK CELL? For example, if you have data in a single column with a blank row separating each data set, how can you have a sum displayed in the separating space if the number of rows between each space is different? Obviously I'm looking for a fuction to automate the task to avoid manual processing.
Thanks!

Very Cool Noob! Thanks

ChemistB

Thanks for the reply. I'll try to be more specific.
I am analyzing sales information for the past year for about 2000 inventory items.
In column "A" I have product part numbers. There is a list of invoice numbers in column "B". There are many Invoice numbers per part number, one number for each time the part was sold over the last year. Column "C" shows the Quantity sold for each invoice.
I want to find the total quantity sold for each part number by adding up the quantity sold information per part number. The problem is that each part was sold a different number of times so my "Sum" range is inconsistant.
There is a blank row separating each block of invoice/quantity numbers, so I thought I could have the formula somehow detect this.
Check out the attachment for a simplified example.
Any thoughts?