Computer will not start after changing permissionl

I was having trouble putting files on my hard drive without signing in each time. I made some changes in permissions and must have inadvertently changed the everyone to No Access, because now my computer will not start up. It gets the white apple on a black background and just goes about a third of the way and stops. Have I permanently locked up the drive. It does not have a cd/dvd drive so I have not start up disk. It was running 10.10.5. The only thing I have is a thumb drive with 10.9 on it. Kink of desperate here and would appreciate any help anyone could offer.

jwm48

iMac (27-inch, Late 2013)