My iMac won't detect an external display

I'm using a mini display-port to hdmi lead and it worked once. I unplugged the hdmi cable when it was still connected to the mac and now it wont find it. Please can anyone help? Thank you in a advance

You can try restarting in Safe Mode. To do that restart the computer, when you hear the startup tone hold down the Shift key until you see a progress bar. A Safe Mode restart will clear some caches, it is possible one has become corrupt. After it has fully restarted in Safe Mode restart normally and test. If that doesn't work then can reset both the SMC and PRAM by doing the following:
SMC RESET
Shut down the computer.
Unplug the computer's power cord and ALL peripherals.
Wait 15 seconds.
Attach the computers power cable.
Wait another 5 seconds and press the power button to turn on the computer.
PRAM RESET
Shut down the computer.
Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.

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