Suddenly can't ssh/ sftp with initially created user
 

Linux server, 12.04 LTS
I followed this guide: http://www.thegeekstuff.com/2012/03/chroot-sftp-setup/
Works great for my sftp user, the user can't access via ssh and only to his directory. However, the user I made when I first installed my server suddenly can't ssh in via PuTTy: it gives an error like " the software caused the..." (I can't remember the rest of the error.) So I made a new user with a home directory and can login just with this new user via ssh and sftp just fine, although all that is displayed for this new user is a $ sign instead of "user@(ip address)", like when I could login via ssh with my first account before the guide. Someone please help?
Thanks!

If it helps, I accidentally removed my first account from the admin group. I thought I added it back. I also can't use the new account- it's not part of the sudoers group, same error. Sorry for the beginner questions, and any help is much appreciated!

Hi,

Check whether the ssh key kept in known_host file at destination host is same as kept at your side, If it is deleted then you might face some problem.

Thanks for the answer. However, due to time I simply reinstalled and reattempted to chroot an sftp account. This time it succeeded without anything going wrong, and now everything is set up as desired. All I can figure is that I broke sudo and though I fixed it, when I didn't. Thanks for the response, though!

I quickly scanned that article. I didn't see anything about mounting the ftpuser's partition with the noexec & nosuid options.
If you use " ChrootDirectory" option to chroot the directory, this is necessary. If the directory is on the root partition, and you can't, look at using a bind mount.