You have made a wiring error when putting in the new light fitting - an electrician may be best.
It is not possible to offer more advice because you do not give any details of the intallations before and after your work.
You may well have wired switched live to neutral - in these circumstances the light will be On when the switch is Off - and the fuse will blow when the switch is set to On.
Peccavi
April 2010