I hate to play devil's advocate, but buying 5000 songs on IMVU was a poor choice. I'm not sure why you'd need that many; their catalogue didn't have a ton of unique or exclusive music. If hosting or DJing was your thing, you'd of been better served and payed less to set up your own stream like many (decent) DJ'S use on VU and SL. Nothing's forever with digital media and at some point VU is/was gonna shut down, in which case you'd lose all that music anyway.
It doesn't make what they did right, but I'm just saying you could've saved yourself some heartache. You should still have access to your playlists as well. That said, you should try contacting customer support for a reimbursement, though I suspect that ship has sailed. I got a semi-decent refund, as did many others back when they limited product sounds to 15 seconds and everyone's trigger music broke. If I recall you had to provide transaction evidence, so you should be able to log the purchases for a refund -- if they issue them at all and it isn't too late. I wouldn't expect more than 25% of the original value however; just how the cookie crumbles with them.
N.B. This is why I'm a little sad we don't have a new means of media storage in stores. CD's don't have enough storage for modern games, so even physical copies aren't forever like their older counterparts. Equally, very few people buy physical now anyway, so companies aren't going to invest in a new standard for a consumer minority ..sadly. It would only drive the already high price up, anyway. I still proudly collect music CD's and vinyl.