Wow! Really freaky! What's this: "but it only gave me the log on screen"? What is the log on screen? What do you see there?
Your sequence of events is really confusing. Did this "but it only gave me the log on screen" problem happen first as it is the first thing you talk about? Or did the Software Updates problem occur first? Totally lost me. What happened first, and what is happening now?
One other thing you mention that leaves me guessing: "not been able to switch it back on". What is the meaning of "not able"? Does that mean if you press the power button nothing happens at all? Does it mean it goes into the startup sequence, but somewhere along the line, it fails to complete? What is the last thing you see on the screen?
For the problem of no response from the power button, reset the PMU. How you do that depends on the computer, and yours is a... ???
As you can tell, I don't have a clue what is happening based on these descriptions. I hope you can provide chronological and detailed descriptions when you submit this question again.
In the mean time, stop wasting your time. Boot to the OS X install DVD. Go past the language screen, open Disk Utility from the "Utilities" menu, select the hard drive at left, select the "First Aid" tab, and repair. If it finds any problems, repair again. When it says "No repairs were necessary", quit Disk Utility, quit the installer, and restart.
If that doesn't fix it, boot to the OS X install DVD again, go past the language screen, click the "Options" button, and select "Archive Install" and put a check/tic at "Preserve Users and network settings".
This is all based on your not getting a proper login window. If you do get a proper login window, but your user account will not login correctly, boot to single user mode and apply the command "mv Library Library-old" to your user Library directory. If you are not familiar with that:
Restart holding ⌘ S.
{You will have a black screen with white text that makes no sense to you.}
When you get a stable command prompt,
Type "/sbin/mount -uw /"
{No, you don't type the ", but be sure to type space when needed.}
Press Return.
{Nothing appears to happen, no problem.}
Type "rm /var/db/.applesetupdone"
Press Return.
Type "reboot"
Press Return.