I have a Dell flat screen monitor+it just won't turn off or on,any ideas how to fix it?

Assuming pressing the power button does nothing, the voltage regulator chip might be busted. Assuming it shows some sign of life (a light coming on) would indicate the back lighting being busted.

Also, be sure you
a) have your computer set to use it as the main display
b) have it all firmly plugged in.

Many monitors don't actually turn off completely, due to the fact that the power regulator still sends power to the monitor. If the screen will not shut off completely after shutting down your computer, depending on what kind you have, then perhaps the On/Off might be damaged.

What is the model number of your flat screen monitor? And how long you've had it? It helps greatly when you provide in-depth of your hardware.

Cheers