For most of us, most of the pictures we take are failures. This is normal. Sometimes, for whatever reason, we conceive a passionate desire to salvage some failed picture. Perhaps the picture is a unique picture of a unique event, or a location not to be soon re-visited. Perhaps we desire the salvage for reasons of mere whimsy.
First: Don't. Reshoot it or toss it. Second: If you must try, don't just apply a cheap effect and hope for the best. Don't tonemap it, apply a vignette, and hope that nobody will notice. Don't just turn the sharpen knob all the way to 11. The picture is bad, you are not going to make it be good.
No. You must re-imagine the bad picture as a new picture, one that is good. If you are successful, the new picture will be a different picture, it will not be the bad one repaired. By this time you have already tried and failed to fix the bad picture, it is unfixable. It is gone. There may rise from the ashes a new picture which is good, however.
First you should get straight in your own mind what is wrong with the bad picture. Since it failed, it does not meet your vision in some way or another. It might be as simple as a technical flaw, the focus is off, it's under-exposed or over-exposed. Possibly it's a more abstract problem. Regardless, get as clearly as possible in mind why this picture isn't a good one.
Next clarify in your mind what must be preserved. Is there a person or object in the picture that must be salvaged? Is there simply an idea that must be saved? Or perhaps nothing at all?
Since the flaws are un-fixable, you have two choices: embrace them, or render them irrelevant. If the picture is out of focus, imagine it as a soft picture. If it is intolerably underexposed, imagine it as a murky piece or a surreal piece. If the colors are un-fixably bad, imagine it as black and white, or as another surreal idea. Can you crop away the problem while preserving that which must be preserved? Experiment. Add grain to an intolerably noisy picture, experiment with radical adjustments and crops, see what the results look like.
Spend some time looking at other people's pictures, keep the results of your experiments in mind. Try to find pictures to look at that bear some resemblance to your experiments. Gather up other people's ideas and let them slosh around in your mind with your bad picture and the experiments and your mental list of what's wrong and what must be saved. Let the whole gestalt ferment a while. Let go of your original conception of the picture, as best you can.
You're trying to pre-visualize in the manner of Ansel Adams and his acolytes, but not a scene, instead a picture you have already made.
At some point, perhaps some re-imagining will occur to you. Count yourself and your picture lucky, go forth, and make that re-imagining. Quite likely, no such re-imagining will occur to you, or no such re-imagining will be successful. Now, having done your best, you may consign the picture to the dustbin.