Report to the police.Get an abstract.Go to the nearest regional KRA office with the abstract,pin cert,I/D and for good measure,carry the original logbook.Apply for the number (Get a receipt).Can't tell of the current cost but the last time,it cost me 3K for both plates.
NOTE:
If it's the rear plate,mark it as "SQUARE" on the application form and if it's the front,mark it as "OBLONG" (I don't know where they got that funny sounding name.It scares customers... )...If you don't get it right at this stage,you might find yourself with plates of the same colour and shape.