I have a Yamaha PW50 which seems to be in good cond but it doesn't start or run can some on please help me out.

First, see what you dont have.
You need good compression
Does it have a good fat blue spark at the plug.
Are you getting fuel to the spark plug.
Any bike that sits for half a year could have bad fuel, and the carburator should be cleaned, as it might be gummed up. Changing the fuel in the tank will also help.