My speakers are plugged into the computer they're power is on and they are'nt working

usuall on speakers it does'nt matter other than both must be the same. If you do connect them incorectly 1+- other -+ you will notice a loss of bass but should not domage the speakers. However are you talking about connection to a power supply?

is it possible that you may have plugged the speakers into the microphone or audio input jack on the computer instead of the speaker/headphone/audio output jack?

Should it be a power supply that outputs AC and converts to DC inside the speaker case, or is it a wall adapter the outputs DC straight to the built in amplifier?
Some speakers have the voltage and polarity for the plug on the back of the speaker where the power lead plugs in. As they are only small speakers of probably a couple of watts it may only need about 2 Amps or less. Higher current is fine as the amp will only use what it needs to work correctly but I would'nt go below 2 Amps.

sounds like the woofer in that one speaker is bad

Led's only work one way, so it does'nt matter what colours they are. The led's that they connect to will only work when the plug is the correct way. You wont do any harm if they are wrong, you just turn them round. Plug them in one way and if the led for the hard drive does'nt come on then it's the other way. Same with the power light. The power switch when pressed just makes a momentary connection across 2 pins, so it does'nt matter which way, it's just like touching the pins quickly with a piece of metal.

You may have an earth loop problem. This is caused by connect various equipment to different mains power outlets.
Connect all the PCs, audio and speakers to a single mains power outlet, use a power board so they are connected to the one mains power outlet.
I suggest you use a power board that has a built-in surge protection.

If you have head set connected to computer the computer speaker will be functioning thorough head set and you will not hear sound from computer. When head set is disconnected the computer speaker will function, just like usual!

https://support.dell.com/support/edocs/acc/ada745/ada745.pdf

That is the setup guide for the speakers.

The two wires plug into a cable that then that single cable plugs into the computer, this way you have sound coming out of the left and right hand speakers.

CHECK THE CONTROL PANAL CHECK THE FOLLOWING FOLDERS

DEVICE MANAGER
NETWORKING AND SHARING
SOUND
WINDOWS FIREWALL & WINDOWS DEFENDER

SYNC CENTER IF THEY ARE PLUG AND PLAY DEVICES

ROD

All speakers should work. Check in the control panel for your sounds & volume settings. If your speakers need a power supply, you'll have to plug that in too.