When upgrading SQL Server there is limitation on the direct upgrade possible i.e. you cannot upgrade SQL Server 2000 directly to SQL Server 2012 or later versions. And you cannot upgrade from SQL Server 2000 to 2008 or 2008R2 if you are not running on SP4.
For quick reference here is the table showing which is the SQL Server version you can upgrade from and also what is minimum requirement of Service Packs even if you can upgrade.
|From\To||SQL Server 2000||SQL Server 2005||SQL Server 2008||SQL Server 2008 R2||SQL Server 2012||SQL Server 2014||SQL Server 2016|
|SQL Server 2000||SP3+||SP4+||SP4+||Not Possible||Not Possible||Not Possible|
|SQL Server 2005||N/A||RTM+||SP2+||SP4+||SP4+||Not Possible|
|SQL Server 2008||N/A||N/A||RTM +||SP2+||SP3+||SP3+|
|SQL Server 2008 R2||N/A||N/A||N/A||SP1+||SP2+||SP2+|
|SQL Server 2012||N/A||N/A||N/A||N/A||SP1+||SP2+|
|SQL Server 2014||N/A||N/A||N/A||N/A||N/A||RTM+|
|SQL Server 2016||N/A||N/A||N/A||N/A||N/A||N/A|