How can I recover large text strings Excel changed into # symbols?

I need to build a CSV file that includes a column of lists of page URLs with space delimiters. The strings average about 70 characters, and in some cases there are as many as 1,000 of them, so a total of something on the order of 71,000 characters including the delimiter space characters.

When there are just a few URLs in a cell, Excel (Office 2007 on Win 7 Enterprise Ed.) handles it just fine. I can see the first URL in the string, and all the rest are there and readable. But some of the rows need to have several hundred or even more than 1,000 URL strings. Excel wouldn’t even allow me to paste these long strings in till I formatted all the cells in the column to Text. It then allowed pasting in the data, but it displayed 255 # characters in a string like #########... Saving the spreadsheet as a CSV, I found that Excel wasn’t kidding. In place of my data, it was putting out 255 character strings of continuous # symbols. It took ages to gather and paste that URL information into the spreadsheet, and the only way to get it again if it is unrecoverable in Excel is to repeat all the searches I did to get it originally.

I do have the spreadsheet with all the apparently corrupted data saved as an XLSX file. Is there any way to recover the corrupted data in it, or must I start over again. If the answer is to start over again, what do I do to avoid the same data corruption on the next try?