Why has it stopped working?

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MS Excel File can get corrupted due to virus attack, missing MS office files, power failure and other possible reason that can be fixed by only use Stellar Phoenix Excel Recovery fix corrupt Excel (.XLS/.XLSX) files and restore everything (including charts, chartsheets, cell comments, worksheet properties etc.

How to solve some troubles with excel files you may learn from- http://www.filerepairforum.com/forum/microsoft/microsoft-aa/excel/481-%E2%80%8Bexcel-files-corrupted

How to Recover an Excel File for Free

1.Check the Recycle Bin--if you've accidentally deleted the file, it should still be there unless you've emptied the bin, and you can click on the file and recover it.

2.Use the Auto Recovery Pane when you open Excel. Review the document recovery pane. If Excel has managed to recover your file, it will show in a separate pane on the left-hand side. Identify your document in the pane, then click on it and choose "Open" to open the recovered file.

3.Try manually repairing the file. Click on "File" in Excel 2003 or the "Office" button in
Excel 2007, then select "Open" and click on the arrow next to the "Open" button. Click on
"Open and Repair" and choose from two options: click on "Repair" if you want to try to completely recover the file, or click on "Extract Data" if you previously tried Step 1 and it did not work.

4.Search for the file manually on your computer. When performing a system search (click "Start" and select "Search" in Windows XP and Windows 2000, or press the "Windows" button and "F" key in Windows 7 and Vista), make sure you choose "Advanced Search Options" and then check the "System and hidden files" check box. You may be able to find your missing or corrupted file using this search: double-click on the file name in the search results to open it.

If nothing can help you, look at Excel Repair Toolbox

As a solution, it costs money, but effective- http://www.excelrepairtoolbox.com/

I don't think you can. It's a real resource hog especially if youre using 2010/13. See below.

Excel 2003 is officially limited to 1 Gigabyte (GB) of memory.
This limit appears to be a limit on the working set memory used by the Excel process, which is the memory reported by Windows Task Manager.
Although Excel 2003 has a substantially increased memory capacity, many of the individual specific memory limits listed below have not changed in Excel 2003.

Excel 2007 is limited to 2 Gigabytes of memory for the Excel process under Windows XP/Vista (Windows memory limit).
This 2 Gigabyte limit is a limit on the Virtual Memory address space. Virtual memory used by a process is larger than the working set memory reported by Windows Task Manager, so the amount of useable memory under Excel 2007 is considerably less than twice that of Excel 2003.
Because Excel 2007 (Excel12) also requires more memory to store the indexes to the increased number of rows and columns you may not be able to load larger workbooks under Excel 2007 than was possible under Excel 2003. Memory Fragmentation may also mean that it is difficult to make use of all of the available 2 GB of virtual memory.

Many of the individual memory limits listed below have been removed in Excel 2007.

Excel 2010 is available in 2 versions: 32-bit (2 Gigabytes of virtual memory) and 64-bit (8-Terabytes (which is 8000 Gigabytes of virtual memory)).
The 32-bit version has the same memory limits as Excel 2007, but the 64-bit version, when used with a 64-bit Windows operating system, will have extremely large memory limits.
Note that the different versions of Windows Vista 64-bit and Windows 7 64-bit support differing amounts of RAM.

Click File > Options > General. Change the default font in "When creating new workbooks"

my yahoo messenger keeps closing (crashinggraying out or sleeping not able to open to do instant messeging

Better Stellar Phoenix Excel Recovery which can fix corrupt Excel (.XLS/.XLSX) files and restore everything (including charts, chartsheets, cell comments, worksheet properties etc.

what happen to me one day i tryed to open windows live on windows 7 and i got the error message that i had a problem that microsoft will get in touch with me when they fix the problem.....so who has the time to wait so i deleted windows live and i went in the registry and deleted all windows live under microsoft and any windows live files...may take a while but it worked for me...i installed the program again and now it works fine

Excel may have "Ignore other applications" selected. See THIS Microsoft Knowledgebase Article.

Try going to Excel Options; Advanced tab; and under the 'General' heading: uncheck the box that says: Ignore other Applications that use Dynamic Data Exchange (DDE).