Log in to wp-admin doesn't work

I have even gone in to my c-panel and changed the user/pswd and still cannot log in to myurl.com/wp-admin on several sites. I have also gotten an error message when I try to preview a post that tells me I do not have permission to view the draft.

I have tried Chrome,IE, and Firefox and still have the same problem.

Any ideas? Thanks for your insight. (I am not a techie…when things work it’s great, when they don’t, I just don’t know what to do.)

Thanks!