Have many odt files. some will open,others will not
Well, you don’t give us much to work with.
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What is your Operating System and which version of LO are you using?
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How exactly have you tried to open the files (from Recent Documents list, OS file manager, menu File → Open)?
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Are the unopenable files distinguished by anything (date last opened, file size, etc)?
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Are your files all stored in the same folder?
Help us out here.
Sorry, your minimalist statement does not allow an answer.
Edit your initial question and let us participate. We need a lot more information, see comment from @ve3oat. Do not write an answer if it is not a solution to your initial question, thank you.
Try out on www.zamzar.com and convert to PDF or alternate format. If files are not opened/converted they probably are corrupt.