the screen on my iMac has gone very dark

Have you attempted to turn up the brightness? First use the brightness control on your Mac's keyboard. If that does not work please restart the computer and do a PRAM reset per the directions in iMac SMC and PRAM reset.

If that does not help please let us know and we can advise some other things to check such as the System Preferences - Accessibility

You can try resetting the SMC How to reset the System Management Controller (SMC) on your Mac - Apple Support

But you'll probably need to take your iMac to the nearest Apple Store Genius Bar for repair.

Thank you. Solved! I realise I knocked it off when I cleaned the keyboard!

Newer iMac have multiple backlights. If it is darker on the top, bottom, right or left side, then one of your backlights have failed and the display needs to be replaced.