Video Stretched

Newbie here so please be patient with me. I've been editing videos. All was great until I put my desktop files into a folder. At this time, prior to exporting, everything was fine. Then the video distorted due to stretching. I took the destop files out of the folder and though this did not resolve the issue 100% it did help some clips that had been edited. About 80% is still stretched. Any suggestions. It is at 16:9 and resolution at 1080 if that helps. I have set and changed resolutions, however, nothing has changed. I've been doing a lot of chatting online and it hasn't gotten me any further. Hoping someone might be able to help me. 

Kindly

Renée