I would like to know more details about the issue.
Are you having issues entering time against some specific projects or all the projects.
Are you able to select these projects in the Timesheet?
Where you able to enter time against these same projects earlier?
Are the time entry fields grayed out for specific projects?
Are you prompted with warning or error messages while selecting projects or entering time?
The above mentioned issues could happen if the user is not part of the Project team. Or, was part of a project team, and was removed from the team. If, there is a Start and/or End date set for the project, and the user is entering time before the start date or after the end date. On some occasions we have noticed similar issues due to corrupt cache and cookies of a browser. You could try on an alternate browser as well.