vivacity 3 not starting

Took head off to drill out broken exhaust stud and put straight back on was running fine befor hand now wont start turns as it should any sugestions 50 cc 2010 vivacity 3 sportline

 

When you say the head I assume you removed the cylinder as well as it's a 2 stroke?
I assume you have checked to see if you have a spark at the plug?
If all you have done is remove the head/cylinder and not disconnected anything else eg wires it should run.
Check to see if you have knocked anything or accidentally pulled a connection off.
It can sometimes take a little while for fuel to work its way through.
Take the plug out and if you have a spark suck up a little fuel in a tube about a thimble worth. Not too much and drop it in the cylinder through the plug hole.
Then replace plug and try starting it.
This can encourage the fuel to come through. If it starts but doesn't keep running try it again but if it doesn't keep running you have a fuel problem and you need to go over everything you have done when you removed the head/cylinder.
The only other thing I can think of is you have damaged or not made sure the piston rings are in the correct position when replacing the cylinder.

 

Gasket? Piston ring broken?
Duff plug?

 

As scuba.
Did you re-use the same gaskets?
Also do a compression test to rule out a broken ring.