Running a bulk action on a query that is based on the User object can alter the User Licence Type of every User in the database to an Analyst type. Having done this it is possible that Users will not be able to login to Service Desk as the number of Anaylst User Licence types will exceed the number allowed by the licence you have applied.
This is logged as Problem 4456, if you are affected by it please report it to your Support provider quoting this reference number.
If you have run a bulk action against your User object you will need to ensure that your User records all have the correct licence type still. The script attached will fix these for you.
To avoid ever seeing this Problem you can just write queries against the individual User types instead - Analyst, End User, Contact and Account Manager and run the bulk actions against these. It's important to remember that Analysts and Account Managers are sub-types of the End User User type.