ok any mechanics on here?

just bought a focus tdi from a guy who said it does not start, just cut out on him and never started since.

can anyone point me what to try common faults it could be ?

never had a diesal before and just unsure what might be causing it not to start,
the chap said who i bought it off it was running and just cut out and wouldent start again.

anyone advize what to test? common faults or what to try ?

thanks