My iPhone 4s just has a black screen. What should I do?

Have you tried a reset ? Press and hold both the sleep and home buttons for about 10 to 15 seconds and the Apple logo should appear. You won't lose any data as this just a reboot. Try it for starters.

The Basic Troubleshooting Steps are:

Restart... Reset... Restore from Backup... Restore as New...

Restart / Reset

http://support.apple.com/kb/ht1430

Try this First... You will Not Lose Any Data...

Turn the Phone Off... ( if it isn’t already )

Press and Hold the Sleep/Wake Button and the Home Button at the Same Time...

Wait for the Apple logo to Appear and then Disappear...

Usually takes about 15 - 20 Seconds... ( But can take Longer...)

Release the Buttons...

Turn the Phone On...

If that does not help... See Here:

Backing up, Updating and Restoring

http://support.apple.com/kb/HT1414

You're welcome.