Excel problem

My Excel for Mac 2011 is refusing to calculate the results of functions.

It just shows the formula instead of the result.

I've checked the forumla options and is set to show the results.
I have no idea whats going on with it. Any clue anyone ?
are there extra settings I shoudl check ?

 

Thanks.

The problem weirdly went away as mysteriously as it arrived. No clue what happened. My system was acting a bit weird after using the mac partition Excel to edit a windows partition xlsx file - at one point I had keyboard strikes entering into two applications at once

Anyhow - all seems well now