I have a Shimano UN55 square BB. Within about a month of buying my new bike the Truvativ crank came loose and is essentially too rounded out to salvage. Any suggestions on an alternative crank to go with the existing BB? I would like to avoid replacing the entire crankset if it isn't necessary. Currently running 175mm cranks.
When a crankset comes loose, stop riding immediately and tighten it. Even going a little distance can ruin the crankset and possibly the bottom bracket spindle. It sounds like in your case, the crankset is toast.
We don't have enough information to say if you need a new bottom bracket or not (the Shimano BB-UN55/56 are standard square taper (ISO) bottom brackets and cost about 10-15 dollars), but you need to combine that with a square taper crankset. Alternatively, you can replace the bottom bracket to another standard rather than square taper (e.g. Hollowtech II) and get other cranksets with that standard.