|Hi Wazza, when making queries such as this, also advise the make and full model number of the device concerned.|
If the computer is a desktop, there may be more than one socket for the monitor,
There will be the original socket, and if a Graphics Card has been added there will be another, possibly 2.
Perhaps you have connected to the wrong socket.
When disconnecting/connecting always power off before doing so.
Try disconnecting the monitor interface cable (both ends if applicable) 2 or 3 times. This will clean/reseat its contacts.
Please advise back on progress.
Good Luck - Keep us posted.