First, note that it's called the Task Bar, not the "Start" Bar.
The first thing to try is to see if it's been shrunk to zero lines high.
Move your mouse cursor to the bottom of the screen and wiggle it up and down
slightly, trying to turn it into a two headed arrow. If you succeed, just
drag it up to make it bigger.
If that doesn't work, try the other suggestions you've gotten here.
