apparently I messed up too much with my permissions and now I'm getting this while trying to use 'screen' again:
/usr/bin/sudo must be owned by uid 0 and have the setuid bit set
If I try:
chown root:root /usr/bin as suggested in some answers, I get:
changing ownership of '/usr/bin': Operation not permitted.
I've found other similar questions/answers, such as: get back sudo on ubuntu but I'm using Putty with Amazon EC2 web services, and they suggest in to reboot into user mode and then root. I don't know how to do that in Putty.
From what I read is kind of serious, so is the only option to detach my instance from the storage volume and start over again?