There's a problem with Pioneer plasmas & Sky HD boxes.
http://www.pioneer.co.uk/uk/content/...statement.html
Don't see why there'd be a problem with a Sky+ box though - the issue relates to HDMI & HDCP.
Have you tried powering the box off & on again at the plug? [Leave it off for a few minutes, then turn it back on at the plug, wait 5 mins at least, then try & take it out of standby].
There have been issues with Sky+ & SkyHD boxes since the "Anytime" update... some people have found the box gets stuck in standby... power cycling normally resolves it, while a temp. fix 'till Sky sort the software issue is to turn off "Anytime". Although if you have a 160 that doesn't have Anytime anyway does it?
EDIT: Ah, OK... you've already tried power cycling. How long did you leave it off, & how long did you wait before trying to take it back out of standby?