Sounds like its out of tune.

Dose the fuel pump work when cranking?

How much advance is there on the dizzy?

Have you changed anything since it was running?

Have you tried pushing the metering unit flap down ONLY by 1-2mm when cranking. You will need a freind to help or a bit of wire bridged on the starter motor if your on your own.

If you keep turing it over and pushing the flap you will flood the engine and end up with wet fouled plugs so you may need to remove them and clean them.