I have an iphone 1 gen and the IOS version 3.1.1 the latest for 1st gen iphone. Suddenly I am not able to slide the slider which unlocks the home screen.
Your phone has likely hung. Press and hold the power button to try to power it off. If that still fails to happen, the other solution would be to hard reset the iphone:
To reset the iPhone, simply press the POWER/SLEEP button and Home button together. Hold them until you can see the boot screen, which is with black background and Apple logo. One point to note is you have to release both buttons at the same time.
This will not erase your data.
If the iPhone fails to work after that, you might have to restore it using iTunes. But I don’t think that’s the case.