Application is Hanging when database upgraded from MS SQL 2000 to SQL 2008 Server R2

Presently we are working in MS sql server 2000,the database perfectly working with our software for retail sales counter application developed with VB 6.0. But now we upgraded to 2008 R2 server. we didn't changed codes and database structure when we upgraded to 2000 to 2008 R2.

Now we are facing the problem is that, if multiple clients (more than three users) accessing the database through our application . while the heavy billing from different users at simultaneously our application hanged. This situation we have to close this applications and reopen it.This time SQL Log file showing nothing.