Data Validation does not work

I have put up a data validation on my spreadsheet. It works fine until today when the droplist list icon is gone. I tried to redo the box list but it does not allow me to select the range. I clicked data validation >>>>>Allow "List">>>> Source. It does not allow me to select the source in other tab. What could be the problem? Please reply. Thank you.

sound strange you can type the information directly in the reference box

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or name the range and then type in the reference box

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It works when I typed the source.

Yes! So strange why this happens..

Back to work. Thank you fredlo2010.

Glad It helped. Try saving all your work, closing all the excel applications, delete the temp folder contents and restart our computer. You will not believe all the garbage Excel can accumulate and stop it from being functional