I had a similar issue where I tossed the BP (I always keep them for just this reason, except this one time...). Wrote MP customer service a letter and included the e-receipt (get it from united.com past trips section), ticket number (as lexdevil suggested, CC statement/calling them you can get it), and pertinent info - dates/flight #'s/MP#, etc. Did you book it on united.com or elsewhere? Had the miles credited within a couple weeks.