Hmm, that is a strange situation; I would remove, any new bulbs you installed and start checking the basics. Obviously, adequate battery voltage is required; assuming, that is ok with a new battery?
When, turning the engine over; does the starter, spin at all? If not, you might check for power @ the starter and see, if you can jump power to the starter, see if it turns over. If with power, the starter won't move; then, you might pull the starter and have it tested. If it turns over; you might check the starter relay and ignition/starter switch, these have been known, to fail and kill the power to starter.
More info here:
Hi guys, I'm a long time poster on these forums but my last how-to on installing an iPod dock was archived so I'm showing up in the forums as a new user. Anyway, my beetle is now long in the tooth and have run into the dreaded ignition-switch short. Symptoms are as follows. Unresponsive...
www.newbeetle.org
starter relay location:
Beetle L4-2.0L (AEG) (2000) > Volkswagen Workshop Manuals > Starting and Charging > Starting System > Starter Relay > Component Information > Locations
workshop-manuals.com
starter replacement;
Beetle L4-2.0L (AEG) (2000) > Volkswagen Workshop Manuals > Starting and Charging > Starting System > Starter Motor > Component Information > Diagrams > Page 6776
workshop-manuals.com
It maybe related to your "bulb upgrades"; on the other hand, you never know, it could have been coincidence and something unrelated, is the problem.
Do some testing, of the basics; report back and we can go from there. Thanks.