SQL Express 2008 backup failing

HI,
I have a Windows 2008 R2 server with SQL Express 2008 installed, we have been using it to run a sharepoint 2010 database for over a year. I have been successfully backing up the database using SQL Management studio. I have recently started getting the following error, any ideas?
TITLE: Microsoft SQL Server Management Studio ------------------------------
Backup failed for Server 'XXXXXXXX\sqlexpress'.  (Microsoft.SqlServer.SmoExtended)
For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=10.50.1600.1+((KJ_RTM).100402-1539+)&EvtSrc=Microsoft.SqlServer.Management.Smo.ExceptionTemplates.FailedOperationExceptionText&EvtID= ...