late 2013 imac won’t start.

hello, i have a late 2013 iMac that gets stuck on the progress bar after booting up. it takes a while to get to the end of the line but after that it stays still. i tried entering safety mode but it won’t enter and on the S.O.S. section on the macos recovery it says there is no problem. i have important documents in there. how do i fix it?