It is possible to use an Access database as a back-end to an ASP.NET application, but according to the ASP.NET Data Access FAQ:
Microsoft strongly recommends against using Access in web applications.
(Click the link above for details.)
If the ASP.NET application will encounter anything more than the absolute lightest of traffic (especially if the Access application truly is "massive") then you would be well advised to convert the Access database to SQL Server and use that as the back-end for your ASP.NET app.