Start out by saving a copy of your database as it is right now.  You hopefully are making back ups any way.

Then run down the list on the Troubleshooter page.

Masih,
Is it that you cannot add a new record to the table?  Or, is it that you cannot add a new field?

A new record.

If it was working before, did you make changes to your form?  Perhaps you set the allow additions to no in the properties.

The other cause you might have changed your data source to where it made the record not updatable.  See the Not Updateable video.

In your original post you said 'all your tables' so the problem probably runs deeper than simply a corrupt table. I would suggest creating a new database file and importing all the objects (tables, forms etc) from the old database to the new one.

Import Objects

Masih,

Are you trying to add a record directly to the table?  Or, are you using a form?  If a form, do I understand that this problem occurs in all forms?

Yes. I am using forms to update. Thanks anyway, I have used now my backup and it is running ok now. But I wanted to know the reason for this since it never happened to me before.

Glad you got it working. Shows that backups can be very useful