All this is assuming that you tried to operate the stereo with the key on and the radio turned to on position.
Which Radio is it? Pop the two pull plugs, at the bottom left and right, looks like two little vertical slots. push them in in and they pop out, this unlocks the radio from it's chassis. make sure the plugs in the back are all firmly seated and that your fuse isn't blown under the hood. Had a cigarette lighter with a fuse blown, looked at everything before i even thought to check the fuse...duh... The picture is a radio just like mine. Nevermind the attachment for the XM radio, i bummed the pic from Google. You see the two little skinny, vertical tabs at the bottom on the left and right? push those in and two little finger holds will pop out, the radio will slide right out, making it the EASIEST radio i have ever seen to come out of a car. Hope that helped a bit. If not, your not far from me, bring it by the house and I'll try to help you with it.
pull the radio out and call the parts dept of your local volvo dealership. they will ask you for probably your VIN and a few numbers off the radio and then they will give you a pass code. sometimes if the radio display says "OFF" you have to turn the key to ON but DO NOT START. put a battery charger on and let it sit for about 2 hrs. then the display will say CODE. enter the code the dealer gave you through the preset buttons. then it will work.