Every WordPress upgrade brings howls of anguish on Twitter and other forums as installations that were working perfectly well prior to the upgrade suddenly crash.
Some people ignore WordPress upgrades in order to avoid the problems - if it ain't broke don't fix it.
But that's not smart because many of the upgrades are designed to plug security holes.
So here's a process for doing WordPress upgrades that will enable you to understand what's causing the problem (if you experience one) and, therefore, what you need to do to fix it.
Before you start
It's difficult to over-emphasise the benefit of having good WordPress hosting.
I use Hostgator because they have excellent support for WordPress, but there are other hosting companies that WordPress recommends - see the list here.
As they say on that page: WordPress works best when it's in a rich hosting environment.
If you're with a hosting company that's not optimum for WordPress, the possibility of problems arising is going to be greater - so you should really move to one that offers good WordPress support.
Process for doing WordPress upgrades
So, assuming you're using a WordPress-friendly hosting company, here's the process I go through for upgrades:
Make sure all your plugins and themes are up to date, and upgrade any that have upgrades available.
Back up your entire site - both database and system files. I wrote more on backing up WordPress here.
Deactivate all your plugins.
Although the WordPress automatic (one-click) upgrade process deactivates your plugins prior to doing the actual upgrade, it also reactivates them all immediately after.
Because of that, if one of your plugins causes a problem you're not going to know which one it is - you'll just see the problem.
So you need to keep control by reactivating them individually and manually.
Any problem that results from a WordPress upgrade is 99% certain to be caused by a plugin. (The other 1% will be a theme!)
Do the WordPress upgrade.
With all your plugins deactivated, there's no reason not to use the one-click upgrade option but, if you're happier doing it manually, the choice is yours.
In a new tab, navigate to your site so you're viewing the front page.
Make sure everything is as it should be, but remember that with all your plugins deactivated any layout features driven by a plugin will not be working.
If there's a problem while all your plugins are deactivated then the cause will be a theme.
So switch to the current WordPress default theme (TwentyTen at the time of writing) and check your site again.
Assuming all is well when you view your site, reactivate one plugin. Then switch to the tab in which you're viewing your site and click the refresh button.
Make sure everything is as it should be and, if it is, repeat the process with each of your plugins in turn. Check your site after each one until they're all reactivated.
If you're running a cache plugin reactivate that one last.
If there's a problem
If you find a problem when you refresh the view of your site, switch back to the other tab and deactivate the plugin you just activated.
Then check your site again by refreshing the screen.
If the problem has disappeared you will know which plugin is the cause.
At this point your best bet is to leave the plugin deactivated. Often the plugin author will issue an upgrade within a couple of days.
However, some older plugins may no longer be supported. In this case you will need to find another way of doing what the plugin was doing for you.
Finally, if a plugin causes a Fatal Error and you get a white screen, you'll need to access your site files via FTP or your hosting company's file manager.
Find the plugin in the wp-content/plugins folder and delete it.
Remember - 99% of problems experienced after WordPress upgrades will be plugin related.
Which is one of the reasons I wrote here about the need to use as few plugins as possible.