Honda Dylan Wont start?

Hi all,

Today is thursday and my honda dylan for some reason won't start. I used it on Monday no problem. Come to start it up today no joy. It started for a slip second but then cut out. It just turns but doesn't fire up. I changed the spark plug cos i didn't know what else to do. Still no joy. I kept trying and trying it almost started but nope, no joy. Any ideas.

Petrol tank is full BTW