I am very new to this & have Lightroom Classic CC
I imported my PNG images into LR, played with them & then exported them as PNG's (using the Magic Export Plug In) ...all was good :-)
I was so pleased with the results that i then replaced the original PNG's with the new ones, as they looked so much better!
The next day while looking inside LR my images looked over exposed & strange ...i tried going back via History but it made no difference so i then deleted the History, but they still looked strange! So i thought i'd just remove them from LR & if i ever needed to i could always re import them to start again.
Out of interest i tried this but when i went to re import them they were all greyed out & won't let me re import at all
Your mistake was to replace the original PNG file with the new ones. This is not a good decision, in fact it is destructive, and not how LR works. Lightroom needs the originals to work with; overwriting the originals with the edited ones simply causes the result you see in LR now, "overexposed and strange".
If you have your original PNGs stored somewhere, put them back, overwrite the edited PNGs. If you don't, then I would imagine the best thing you could do now would be to remove the PNGs from Lightroom, and then import again.
I had a feeling that was my mistake ...great, & thanks very much for the quick response! :-) :-)