The first thing to check is the week being shown in your timesheet. This is shown in the top right corner of the timesheet:
It will show either a date, or a label, for example “This week”, “Last week”, or “Next week”.
If you need to change the week shown to be able to enter your time, click on the small downwards pointing arrow next to the week text and a drop down menu will appear. From here you can select any specific week that you need to log time in and then click "OK":
