Odd 'Save' Bug, tries to Save to New nonsense File

Does anyone know why, when I attempt to Save, I get the Excel error message:
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"Microsoft Excel cannot access the file 'G:\Health\Cancer\Detection & Monitoring\2349D200'. There are several possible reasons:
The file name or path does not exist
The file is being used by another program
The workbook you are trying to save has the same name as a currently open workbook.
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Note:
1: I'm trying to save the spreadsheet as normally, via clicking on the disk icon at the top
2: I've just opened this spreadsheet and only altered one cell (of about 500,000)
3: I've already just saved successfully when I altered 2 other cells
4: There is no other spreadsheet opened.
5: The spreadsheet is now 'dead' - unresponsive to any action; can only be closed via Task Manager.
6: It seems to be only when I do one specific cell change - LN3 - and the file that Excel claims not to be able to access changes, for example to 69EB000
7: This error is sporadic
8: Running cleaning programs (TuneUp and AVG) and rebooting has no effect
9: At the moment, perhaps because I no longer attempt to change LN3 (which I have now successfully changed, by saving after each trivial change), all seems well.

Any ideas please?

Thanks for the idea xladept; I wasn't even aware of the 'repair' option.

However, in this case, I cannot get that option with my spreadsheet. Following the instructions for 'Repair' from Excel Help, I'm told that I will be presented with Open and Repair from the Open option - not so in my case. Presumably because my spreadsheet is not seen as corrupted. Perhaps I have to wait until the fault re-appears. So I'll bear it in mind.

OK. That I understand. Thanks for making it clear.

At the moment my spreadsheet is working reasonably well, and I have a bunch of other stuff to do. But when I've time, and/or I get a repeat of this bug, I'll hit that 'repair' on the re-installation

Thanks for your guidance.