Excel 2016 - Cannot type anything into a cell all of a sudden

For my side, most of the files are about less than 5MB or so. Admittedly a few of them are relatively huge like 20-30MB in size. But I honestly doubt it is the size that is causing the issue.  It works ...just that sometimes it will not allow the user to input data. The solution is to switch to another workbook and back again, and then it is fine. There doesn't seem to be a pattern as to when it will happen. Some of the users said that it might happen like once per week then in one day, 3 to 4 times (or even more). Very frustrating for the normal users who expects their Excel to just work. Especially if the IT (like me) tells them that they are running the "latest" and "greatest" version of Excel.

I think Excel might just be too bloated with too many features. So much so that Microsoft can't be properly debug their software.

IT professional here. Recently migrated to O365 Pro Plus for >100 of our users. Can confirm that the recent updates to O365 has completely ruined Excel's reliability. From display issues, to crashing (even after full reinstall/reimage of the machine), the most annoying problem we encounter now is with anyone with more than one spreadsheet open. It causes issues with entering data, and printing. Had a user recently that tried printing 3 different documents that all never made it to the printer until she switched windows to the first Excel document that was opened that day where Excel decided to queue up all the jobs at once. 3 users today have had the issue described in this post topic. All versions are on the most current public release. I cannot express my frustration enough. Would expect some kind of acknowledgement on M$'s part regarding this widespread issue, but of course there is not a single KB article on the issue which has persisted for numerous users for at least 6 months.

Thanks everyone for setting this straight.  Most of my files are less than 1MB and this still happens. I have to admit though it is completely unpredictable and wish it was as simple as restarting my computer.

Your workbook is too large.  Try to divide that workbooK into two.  Example: If you have a years worth of data, split that up into 6 months.

Kjome, are you serious? We are talking about excel files, files that worked just fine in previous Windows or Excel versions and not some rocket science data with, I assume, large files.

The scary thing is, that this is not the only problem with Excel that I am having. Sometimes the cells are not calculated correctly, the other times the graphs do not show the latest change that I had done to the cells...and the next day, everything works fine.

From what I know, the size of those excel files are not that huge. 

The largest I have seen the users using is about 15MB.

Anyway I have seen it happen on files which are pretty small like 2MB or so.