Update your post to include full system hardware specs and OS information.
Include PSU: make, model, wattage, age, condition.
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Power down, unplug, open the case.
Clean out dust and debris.
Ensure by sight and feel that all cards, connectors, RAM, and jumpers are fully and firmly in place.
Refer to both the applicable motherboard and GPU User Guides/Manuals to verify that all physical connections are correct and that the necessary configuration settings are correct.