iMac stuck on “checking fsroot tree”

My iMac wouldn’t load any apps so I restarted it, then it wouldn’t boot up after the restart, the Apple logo with loading bar underneath would just stay at full and not do anything else...after looking online iv tried to run first aid on my hard disks, bottom to top as instructed but now it’s stuck on “checking fsroot tree” while I’m trying to run first aid on the first drive...it’s being like this for over 40mins now....can anyone help, what should I do???