Assuming the dial is not turned down and the 12V battery is good.
The speedometer needs to be resoldered/repaired soon. You've 3 avenues, the dealer [Shop time + new part] (>$1000), take it to a car shop [Shop time + repair] (~$400), or DIY [Your time + repair] (~$150). Do note the "repair" will take place online via Ebay, but the "replacement" would be done as a separate job. To replace the speedometer someone needs to gently take the dash apart and put it together again. The guys on Ebay offer two repair services exchange and repair. If you care about accurate mileage then opt for repair, but realize you will not have a speedometer for a few days and the dash must be opened twice. If mileage accuracy isn't too important then opt for exchange, and they'll trade you a working speedometer with similar mileage. Ebay search: 2004 prius speedometer repair, or 2004 prius speedometer exchange.
IMHO: Pursue an Ebay exchange then pay someone you trust to cautiously do the work for you. It took me ~5 hours to cycle the dash, but I also vacuumed all the accumulated dirt/dust.