If you computer has changed you may have lost your program settings for Discover and other programs stored in -
C:\Users\USERNAME\AppData\Roaming\
This means your list of known drillhole project will be lost.
Solution
You can import any drillhole project from its folder to add it to the list.
Select Drillholes>Open>Import Project.
Browse to the folder the the drillhole project was stored in and select the *.dhproject file to import. If you have forget where you had saved these, you can search in windows explorer for *.dhproject file types.