Can anyone help with importing .xls files to CRM? It has worked previously

Can anyone help with importing .xls files to CRM? It has worked previously

I am using the free version of CRM with 9 users.

About 8 weeks ago I managed to import successfully 250 records from an Excel file.  Today I cannot get the import feature to work and there are no error messages.  I have tried .xls and .xlsx  Both files have less than 1000 rows and are less than 5MB.