I can't find any info on your modem supporting UPnP. If it does, make sure it's enabled within the modem & ticked within LW's tools > options > advanced > firewall config. I don't think it has an internall firewall, so that shouldn't be a problem. Are you using a router as well? If you are, we'll need the brand & model.

Did anything at all change before you lost connection? Any upgrades, change of hardware, letter from your cable company saying they were making changes?

It would be worth it to call your ISP & find out if they've started shaping bandwidth or filtering p2p in any way. Your default port for LW's not blocked but it just sounds a bit fishy. I might be wrong