What graphics card do you have? Buster ships with newer drivers and it is possible that:

1. You card is now old enough that support for it has been moved to the legacy driver package
2. Documentation claims that your card is now old enough that support for it has been moved to the 
legacy package, but despite these claims in the documentation that is not the case.

I had (2) happen to me during upgrade, i.e. documentation said that my card is now supported by 
the legacy driver, but after spending hours on trying to get graphics running it turned out that 
this is not the case and my card is still supported by the main driver.

Janek