This just happened to me on my mule account a few hours ago.
SE emailed me a new password, so check the email thats on the account. Or you may have to contact them if you didnt get an email.
Edit : also make sure you check the junk email folder because thats where mine ended up.
Check your email. SE changed a ton of peoples passwords and sent them an email. Check your junk box too, might have gone there. I had this problem this morning.
ya waiting for the email. just mainly wondering who else is having this problem. lol