VBA string functions not recognized in64-bit Excel

Yesterday I updated my MS Office suite to the 64-bit version. Today an application that uses Excel that I have been maintaining for nearly 30 years will not run. It throws "Library not found" errors for the string functions Left, Mid, Right and others. I interprest this to mean some library has not been loaded, but I can find no informatin on what specifically I should do to get back to where I was yesterday. Any help would be appreciated.
mike sumanRetiredAsked:
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NorieAnalyst Assistant Commented:
Have you checked under Tools>References...?

If there is anything marked as MISSING note the name, uncheck it, close the workbook and re-open then try the code again.

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mike sumanRetiredAuthor Commented:
To Norie: Yes, something WAS marked as missing (actually the second workbook opened, which seems odd) and on deleting that and restarting Excel the problem went away. I certainly thank you for that.

But I don't really understand what is going on. It took several tries to get both workbooks opened because opening the second usually closes the first, which is why I went over to the 64-bit Excel in the first place.  

To Rahul Yadav: Thanks. I guess I understand what you say. But it doesn't seem right that the problem just occurred one day after nearly 30 years when nothing I had done changed anything. I'm getting terribly tangled in what I think are MS problems, and I'm afraid I am over the hill.
NorieAnalyst Assistant Commented:

Can you recall what i was that was marked as MISSING?
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