Unfortunately there's no real way to fix the seal once it's gone - that moisture is trapped between the panes because the seal failed, and there's no way to reseal it from the outside. You've got a few options: replace just the glass panes if your window frame is still solid (this costs less than a whole new window), replace the entire sash on older windows, or go for a full window replacement if the frames are damaged too. I'd get a couple quotes from local window companies since prices vary a lot depending on your window type and size. The longer you wait the more the moisture might damage the frame, so I'd deal with it sooner rather than later.