My V50 came up with a message 'Power unit in need of urgent attention'.

Hi, the same thing happened to me last year, it is the alternator that needs replacing. The car shuts down one system at a time in order to keep it running on a low battery, until it eventually shuts down completely. A new alternator, and a charge to the battery, and all was well again. Only thing is that Alternators on the Volvo V50 can be quite expensive.

Exact same thing just happened to me in the exact same sequence. Alternator. Because I let the car die completely (like you did), I also fried the battery. Looking at $1000 or there is a cheaper less trusted alternator that will put you at about $650 out the door. Good luck.