does your car still have the stock uppipe? do you have a boost guage? if your getting 0psi boost then you might have sucked a chuck of cat into your turbo. if your building some boost but it not alot like only six psi then you most likely have a cracked vac line.
DIY: VF34 turbo on WRX (with pictures!) - ClubWRX Forum - Subaru Impreza WRX and STi Community and Forums
follow these directions to remove your turbo. when you get it out see if its seized. see if theres cat keeping it from spinning. ok so here is where the choice to upgrade comes into play. if you can safely remove the cat from the turbo and there is no shaft play and or other damage then all you have to do is replace the uppipe (180)and bolt your turbo back on. if you want to replace the turbo then you have alot more work to do. there are many direct bolt on turbos. aps, fp, deadbolt, ihi vf series just to name a few. when you upgrade the turbo you will have to upgrade the injectors(700), turbo(700-1300), uppipe(180), em(700), fuel pump(180). before you know it you have spent 3k on your car after tunning(120 an hour minimum of 4 hours) and dyno(60+) to see how much whp the new improvement is.