Users not able to login but present in user list

Hi :

We have couple of users in user list , among those some of the users are not able to login . When i went to SU01 that users are present .

when i try to display/ change the users which is saying that user does not exist . but that users are present in user list .

How to activate those users instead of recreate .

This is very critical for me please help .

Thanks in advance,

Chimsi