Did it work before or is this a new (or used) monitor? Have you recently moved your computer. Are you using a tower PC type or is this a laptop?
If itÂ´s not a laptop:
I think you should first check all connections...unplug each cable including the power, and put them carefully back...wriggle them a bit to make sure they are properly seated.
If that doesnÂ´t fix it:
Take your computer to a friend, or borrow a monitor from someone. Try plugging your computer into the other monitor...this way you can test if there is a problem in the computer or if your monitor is causing the problem.
If itÂ´s your monitor then it may be dead or dying...i.e. time for a new one.
Let us know what happens.