The plugins/widgets were working fine until, I think, a few days ago but now something seems to have gone wrong - and I have NO idea WHAT

I am using Ata v3.4.9 with right and left sidebars each containing about 4 to 5 widgets.
All were fine but now
* calendar - is there but posted days not highlighted
* categories - "select a category" visible but nothing to select
* sitemap - empty
* collapsing archives - completely disappeared apart from the title
* recent comments - says "no comments" - not true
* who's online - always "0" - not true
* contact form - clicking on contact form link returns a "Not Found"

I am totally lost.

The only changes I've made in the last few days are:
* making 1 post
* upgrading about 4 plugins (can't remember which ones though)

The strange thing is, they seem to come back to life when I'm LOGGED IN...
I haven't tested them all, but archives and comments are definitely back and VISIBLE when I'm logged in. Log out, and they're gone...
No good for non-WP-using visitors...

Please help

sounds like you have a plugin that requires you to be logged in
1) start up two browsers, like Firefox and Safari
2) log into your site in Firefox, and visit it without signing in Safari
3) in Firefox, disable all plugins
5) in Safari, check to see if the widgets show up

if they do, activate a couple plugins at a time and check to see it the widgets show up. When they don't, you will know what plugin is causing the issue.

Atahualpa is now at version 3.7.1 you may want to update

THANK YOU !!!
I followed your instruction and it worked!
The culprit was the "Visitor Maps and Who's Online"

I couldn't be bothered to figure out WHY it went wrong, so I simply deleted that plugin - and my blog is working OK again. Problem solved!

Yes, I would love to upgrade my Atahualpa - it's just the worry that something could go seriously wrong by upgrading that is holding me back (a simple plugin upgrade causing a mess like this one... can't imagine what I'd do if Ata upgrade collapsed)...hopefully I will in a couple of months time after I've done enough research

Thanks again!

You can have both versions installed at the same time and activate the latest and if you have problems, swap back to your original.

As always, do a database backup before making any major changes to your site.

THANK YOU for all your help, juggledad

Will definitely try upgrading Ata that way
(Might end up being back here soon...)