Sounds like a Windows issue. Open the PAINT program by searching for it in the Start Menu entries.
Then click FILE | OPEN FILE and choose your PNG. You can use FILE | SAVE AS to save it in another format.
If that's not the fix, share the file and someone will check it out. Upload the file/s to a free server, such as Google Drive or MS OneDrive, get a shared (public) link, copy the link and paste it here or to me in a Private Message via the mail envelope in the top-right corner of this forum. When using Google Drive, if necessary change "restricted" to "anyone with link can view."