Is there a fault on the line?
'Guasto' — GWAS-to. The 'gu' before 'a' is like 'gw' — a labio-velar glide.
Ask when service has stopped or become severely irregular. This distinguishes a technical failure from a strike or planned closure.
'Guasto' as a noun means fault, breakdown, or malfunction. As an adjective it means broken: 'L'ascensore è guasto' (The lift is broken). 'Sulla' is 'su + la' (on the). Knowing the difference between 'guasto', 'sciopero', and 'lavori' (maintenance works) helps you decide on an alternative route.
La linea è sospesa?
Is the line suspended?
Asks the practical result rather than the cause.
Ci sono lavori in corso?
Are there works in progress?
Planned maintenance closures are common on weekends.
Quando riprende il servizio?
When does service resume?
Key follow-up question once you know there is a problem.
When the metro breaks down, Italian transport authorities provide 'bus navetta' (shuttle buses) following the metro route above ground. Look for signs saying 'Servizio sostitutivo con bus' at the station exits.