This is behaviour I haven't experienced before. I'm using Excel 365. The worksheets I'm working on will suddenly become inactive, i.e. fail to respond to most operations, such as selecting a cell, row or column. I can temporarily cure this by selecting another worksheet tab, or maximising the window and restoring it. But then, after a minute or less, having perhaps activated another application and returned to Excel, it will be semi-dead again.

The only other possibly relevant info I can think of is that when copy/pasting cells earlier I had a message along the lines of 'There is a problem with the clipboard...but you can continue pasting'.

It's now making work almost impossible. Can anyone advise how to fix or work around this please?

(I have restarted Excel, with no change.)