These things have been known to just 'crash', so make sure that you have your radio code and then try leaving the battery unhooked overnight. If you prefer not to disconnect the battery, then removing the fuse might do the trick. I'm not sure which one it is, but my money's on the one that's hidden under the ashtray.
The mentioning of the fuse under the ashtray was inspired!
This morning on my Laguna II I wanted to change the CD. However, having ejected (pressing twice as ignition off if that's relevant), it wouldn't take a new CD. Also in the week previous to this the GPS seemed to take longer to react once I'd acknowledged the flaming annoying Highway Code message, sometimes complaining that there was no CD - nothing I could quantify in terms of time, but nonetheless just the feeling that something was beginning to go wrong!
I poked and prodded the eject button a bit on the GPS CD unit in the glove box, presented the CD to the drive a number times, but to no avail. Then I noticed that the screen in the middle of the dash had gone black. In fact I had no display at all - no time, no temperature, and no radio display even though the radio (and music CD) still worked.
The 20 minute automatic switch off of the the radio also doesn't reset things as the screen was blank and the GPS dead at lunchtime as well as later during the drive home.
So this evening, armed with Horatio's punt I removed the fuse from under the ashtray, waited a bit to make sure all residual power had ebbed away, and carefully put the fuse back (I wonder how many have been dropped and lost behind the lighter!?). And it all worked. As Horatio said, you will need your 4 digit radio code.