There's no output in the monitor and then a long beep comes after starting the computes

The long beep may be an error indicating that something is wrong. Unfortunately, if nothing comes out on the monitor, you're not sure if 1) the monitor is bad, 2) the cable is bad or not connected or has a bent pin in one end or 3) the video card is bad or 4) the CPU is throwing a major error or 5) something else is wrong.

Do you have any more information? Did you do something to the computer (insert memory, other hardware, install some program) just before this happened?

If you have access to another monitor, try and hook that up to see if that would work

And as mentioned above, any changes since it worked las

If you have a video card in the computer, check to see if you also have onboard. You can find that near where you plug in your mouse and keyboard. Try to plug your monitor in there and see what happens. It's usually not the monitor unless its an older monitor close to 8 or more years, but that doesn't mean that a monitor that is 2 yrs could go bad. It's just the chance are slim.

Another thing could be your system memory. If you have 2 sticks pull one out and try that. If you get the same results pull the other out and put the first one back in and try it again. If you still get the same results try to reseat any cards you have like a dial-up modem. Sometimes a static charge can be built up in there and cause problems.