Upgraded to 2.9.x, Used Automatic, All OK, but now can’t log on. Error 500

I decided it was time for a tidy up on one of my blogs. Didn’t do anything major, just clicked on a couple of pretty standard plugins to upgrade them – and everything seemed fine. But at the top WordPress was nagging me that v2.9.x was available, so I clicked it. I then chose for it to upgrade automatically and it said everything was fine.

But somehow it had logged me out. I tried to login again and I get: http 500 internal server error.

The website is running OK.

Where do I start to fix this?

Thanking you in anticipation.