Updated Jetpack this morning – now can't log in

Suddenly I can’t sign into my weblog Intheboatshed.net .

When I try to do so using my usual name and password, all I get is a blank screen.

The site shows no sign of having been hacked that I can see so far.

I updated Jetpack this morning. Could that be the source of the problem, or is it more likely to be something else?

If it is relevant, is it possible to roll back my Jetpack install using FTP please?

Thanks,

Gavin Atkin