I started upgrading the Forum from 5.4 to 5.4.2 (and I’m already on WordPress 3.9) when I got this message:
There is a problem with the JavaScript being loaded on this page which is stopping the upgrade from being completed.
The error being reported is: undefined is not a function
My website forum is now effectively out of commission with a maintenance message on the Forum page.
Any thoughts? I have limited knowledge of coding so it would be wise not to assume too much knowledge on my part.
I am afraid it simply does mean that you have a serious enough javascript error happening on your site that the upgrade script just can not run. It almost certainly means that something has caused the jQuery library to stop functioning completely which will happen when a serious enough error in thrown.
This is pretty rare within the WP Admin – more commonly would see it on the front-end of a site.
Assuming all you have done is upgrade WordPress to 3.9 and Simple:Press to 5.4.2 (that means 5.4.2 code even if not yet upgraded) – and have added nothing new into the mix then it is most likely – but by no means definitely – a WordPress plugin that has yet to be updated. People generally (and I do not mean just Simple:Press users here) are seeing a lot of issues with the WP update simply because so many scripts have changed and many plugin authors seem to have not got their plugins ready and updated.
If at all possible – you may be able to force the upgrade to run by deactivating other WP plugins and switching to the wordpress default theme for the task. And then reactivating etc. It’s a pain though and you would want to use Maintenance Mode while doing it.
Or – just deactivate plugins one at a time until things work.
I am willing to take a look for you but it would mean allowing me access as a Simple:Press admin to your site so that I could make the upgrade and use the script console to see if I can find the problem. That is up to you of course. If you think that is best then please PM credentials to both myself and to ‘Mr Papa’. Please include in the PM a link to your site, a link to this thread and brief reminder of the problem. And one of us will take a look for you.
Thanks for your prompt reply
I’ll take a shot at deactivating some plugins and get back to you with the result.
Regards
I am raising a glass of fine Australian red in your general direction.
It was a plugin called Freecounter!!!!. I picked it at random, deactivated it and the Forum is back on.
Kudos!
Wow that is great news!
Well – I am just about to go out for lunch so I’ll tip my glass to you in a little while. Only mine will be an Italian red 
oh, I could go for an Italian red tonight… hmmm… might have to stroll down to that little Italian place a bit from the hotel for dinner..