I encountered a strange thing on all my websites running silverstripe. Today I could not login to them and I always have remember me on this computer checked. I tried my password that I have on the site but it did not work. I also had collegues that could not login anymore. I had to use the link that resets the password.
Have anyone else had this problem. No sites where hacked or changed.
Is your server on a shared host? could it be that something changed on the server side (rather than in the code if you haven't upgraded the site). Commonly something changes which changes how PHP hashes a strings on a particular site.