Lister, funnily enough - that's almost exactly what happened to me. T'was on my first solo X-country (a few more moons ago than yours...) and about 30 mins into the flight the
top latch popped open, somehow folled by the bottom one. Got out the POH and tried to follow procedures - in vain. The
thing simply wouldn't close. Aborted X-country and returned to base. CFI somewhat perplexed asked why I didn't just carry on to my first stop, as the door would just trail w/o doing anything bad. In fact he demonstrated that on a quick flight (in a way I of course knew already, having flown INXS of one hour with the door open! - but tell that to a student
like me at the time....).