There are various kinds of Campanula, or bellflower, and some of the varieties do best with some support provided early on, as they are getting going. I am assuming you have a taller variety. Many tall flowering perennials with large-ish blooms up top will topple or split open without a little help, particularly if you are feeding a lot (too much soft tissue is produced) or they are wanting more sun (weak growth, as if they are stretching for the light, occurs). It’s hard to rectify floppy tall plants once they’ve flopped–again better to stake them next time when they are developing. If you have messy dead flowers on floppy stems, best to deadhead (remove spent flower stalks) or give the plant a more serious early-summer haircut if it’s really a mess.