SQL Server 2005 table loses primary key

Hi All,

I'm using SQL Server 2005 Standard with SP3 and today I discovered that our system send a exception saying that cannot insert null values into one table,  so I went to the SQL managment studio and found out that the table's primary key is lost !!!, this primary key was configured as a unique automatic value, why this happened?, has anyone had the same problem?

I'm really worried about this, so any information would be appreciated.

Thanks

G.Waters